A recent change to xdg-shell-client-protocol causes a build failure.
This was reported upstream as GH #5.  User @AlexBMJ posted a two-line
fix in that issue, but the developer has not yet responded:

progandy/wdomirror#5 (comment)

wdomirror: fix build failure by applying patch from GH #5

Without this change it segfaults when trying to play any media:

  $ jellyfinmediaplayer
  Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome. Use QT_QPA_PLATFORM=wayland to run on Wayland anyway.
  libpng warning: iCCP: known incorrect sRGB profile
  Logging to /home/bf/.local/share/jellyfinmediaplayer/logs/jellyfinmediaplayer.log
  Cannot load libcuda.so.1
  Segmentation fault (core dumped)

The backtrace shows pipewire being at fault:

  $ coredumpctl debug
  [...]
  Program terminated with signal SIGSEGV, Segmentation fault.
  #0  0x00007f711428c9bb in core_event_demarshal_remove_id () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  [Current thread is 1 (Thread 0x7f6ffdc87640 (LWP 1360949))]
  (gdb) bt
  #0  0x00007f711428c9bb in core_event_demarshal_remove_id () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  #1  0x00007f711428886c in process_remote () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  #2  0x00007f7114288e68 in on_remote_data () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  #3  0x00007f7114310efe in loop_iterate () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/spa-0.2/support/libspa-support.so
  #4  0x00007f71266fe7f2 in do_loop () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/libpipewire-0.3.so.0
  #5  0x00007f7128b08e86 in start_thread () from /nix/store/ayfr5l52xkqqjn3n4h9jfacgnchz1z7s-glibc-2.35-224/lib/libc.so.6
  #6  0x00007f7128b8fce0 in clone3 () from /nix/store/ayfr5l52xkqqjn3n4h9jfacgnchz1z7s-glibc-2.35-224/lib/libc.so.6
  (gdb)

Standalone mpv doesn't segfault (when directly playing the underlying
media files). I don't know why.

Fixes: b97cda7 ("mpv-unwrapped: 0.34.1 -> 0.35.0")

Fixes NixOS#205141

Ref jellyfin/jellyfin-desktop#341

Without this change it segfaults when trying to play any media:

  $ jellyfinmediaplayer
  Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome. Use QT_QPA_PLATFORM=wayland to run on Wayland anyway.
  libpng warning: iCCP: known incorrect sRGB profile
  Logging to /home/bf/.local/share/jellyfinmediaplayer/logs/jellyfinmediaplayer.log
  Cannot load libcuda.so.1
  Segmentation fault (core dumped)

The backtrace shows pipewire being at fault:

  $ coredumpctl debug
  [...]
  Program terminated with signal SIGSEGV, Segmentation fault.
  #0  0x00007f711428c9bb in core_event_demarshal_remove_id () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  [Current thread is 1 (Thread 0x7f6ffdc87640 (LWP 1360949))]
  (gdb) bt
  #0  0x00007f711428c9bb in core_event_demarshal_remove_id () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  #1  0x00007f711428886c in process_remote () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  #2  0x00007f7114288e68 in on_remote_data () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/pipewire-0.3/libpipewire-module-protocol-native.so
  #3  0x00007f7114310efe in loop_iterate () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/spa-0.2/support/libspa-support.so
  #4  0x00007f71266fe7f2 in do_loop () from /nix/store/nhffrd7f15dhfbkwzgayq7hhzmdvdy19-pipewire-0.3.63-lib/lib/libpipewire-0.3.so.0
  #5  0x00007f7128b08e86 in start_thread () from /nix/store/ayfr5l52xkqqjn3n4h9jfacgnchz1z7s-glibc-2.35-224/lib/libc.so.6
  #6  0x00007f7128b8fce0 in clone3 () from /nix/store/ayfr5l52xkqqjn3n4h9jfacgnchz1z7s-glibc-2.35-224/lib/libc.so.6
  (gdb)

Standalone mpv doesn't segfault (when directly playing the underlying
media files). I don't know why.

Fixes: b97cda7 ("mpv-unwrapped: 0.34.1 -> 0.35.0")

Fixes NixOS#205141

Ref jellyfin/jellyfin-desktop#341

(cherry picked from commit 3c528bc)

Pull in _FORTIFY_SOURCE=3 stack smashing fix. Without the change on
current `master` `rtorrent` crashes at start as:

*** buffer overflow detected ***: terminated
                                                                                        __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
44      pthread_kill.c: No such file or directory.
(gdb) bt
    #0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
    #1  0x00007ffff7880af3 in __pthread_kill_internal (signo=6, threadid=<optimized out>) at pthread_kill.c:78
    #2  0x00007ffff7831c86 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
    #3  0x00007ffff781b8ba in __GI_abort () at abort.c:79
    #4  0x00007ffff781c5f5 in __libc_message (fmt=fmt@entry=0x7ffff7992540 "*** %s ***: terminated\n") at ../sysdeps/posix/libc_fatal.c:150
    #5  0x00007ffff7910679 in __GI___fortify_fail (msg=msg@entry=0x7ffff79924e6 "buffer overflow detected") at fortify_fail.c:24
    #6  0x00007ffff790eea4 in __GI___chk_fail () at chk_fail.c:28
    #7  0x00007ffff790ea85 in ___snprintf_chk (s=<optimized out>, maxlen=<optimized out>, flag=<optimized out>, slen=<optimized out>, format=<optimized out>) at snprintf_chk.c:29
    #8  0x0000000000472acf in utils::Lockfile::try_lock() ()
    #9  0x000000000044b524 in core::DownloadStore::enable(bool) ()
    #10 0x00000000004b1f7b in Control::initialize() ()
    #11 0x000000000043000b in main ()

Since ba83271 the build fails with

    applying patch /nix/store/46rxbbvl2l3mrxb50y9rzy7ahgx0lraj-d741901dddd731895346636c0d3556c6fa51fbe6.patch
    patching file tests/hazmat/primitives/test_aead.py
    Hunk #1 FAILED at 56.
    Hunk #2 FAILED at 197.
    Hunk #3 FAILED at 378.
    Hunk #4 FAILED at 525.
    Hunk #5 FAILED at 700.
    Hunk #6 FAILED at 844.
    6 out of 6 hunks FAILED -- saving rejects to file tests/hazmat/primitives/test_aead.py.rej

Without the change `unnethack` startup crashes as:

    (gdb) bt
    #0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
    #1  0x00007f734250c0e3 in __pthread_kill_internal (signo=6, threadid=<optimized out>) at pthread_kill.c:78
    #2  0x00007f73424bce06 in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
    #3  0x00007f73424a58f5 in __GI_abort () at abort.c:79
    #4  0x00007f73424a67a1 in __libc_message (fmt=fmt@entry=0x7f734261e2f8 "*** %s ***: terminated\n") at ../sysdeps/posix/libc_fatal.c:150
    #5  0x00007f734259b1d9 in __GI___fortify_fail (msg=msg@entry=0x7f734261e2df "buffer overflow detected") at fortify_fail.c:24
    #6  0x00007f734259ab94 in __GI___chk_fail () at chk_fail.c:28
    #7  0x00000000005b2ac5 in strcpy (__src=0x7ffe68838b00 "Shall I pick a character's race, role, gender and alignment for you? [YNTQ] (y)",
        __dest=0x7ffe68838990 "\001") at /nix/store/B0S2LKF593R3585038WS4JD3LYLF2WDX-glibc-2.38-44-dev/include/bits/string_fortified.h:79
    #8  curses_break_str (str=str@entry=0x7ffe68838b00 "Shall I pick a character's race, role, gender and alignment for you? [YNTQ] (y)", width=width@entry=163,
        line_num=line_num@entry=1) at ../win/curses/cursmisc.c:275
    #9  0x00000000005b3f51 in curses_character_input_dialog (prompt=prompt@entry=0x7ffe68838cf0 "Shall I pick a character's race, role, gender and alignment for you?",
        choices=choices@entry=0x7ffe68838d70 "YNTQ", def=def@entry=121) at ../win/curses/cursdial.c:211
    #10 0x00000000005b9ca0 in curses_choose_character () at ../win/curses/cursinit.c:556
    #11 0x0000000000404eb1 in main (argc=<optimized out>, argv=<optimized out>) at ./../sys/unix/unixmain.c:309

which corresponds to `gcc` warning:

    ../win/curses/cursmisc.c: In function 'curses_break_str':
    ../win/curses/cursmisc.c:275:5: warning: '__builtin___strcpy_chk' writing one too many bytes into a region of a size that depends on 'strlen' [-Wstringop-overflow=]
      275 |     strcpy(substr, str);
          |     ^

I did not find a single small upstream change that fixes it. Let's
disable `fortify3` until next release.

Closes: NixOS#292113

This adds some extremely helpful and popular encoders in by default:
* openjpeg
* celt
* libwebp
* libaom

On the `master` branch, closure size for ffmpeg-headless went up 18.5 MiB.
```
$ nix store diff-closures nixpkgs#ffmpeg-headless^bin .#ffmpeg-headless^bin
celt: ∅ → 0.11.3, +168.4 KiB
ffmpeg-headless: +70.0 KiB
giflib: ∅ → 5.2.2, +398.7 KiB
lcms2: ∅ → 2.16, +466.2 KiB
lerc: ∅ → 4.0.0, +840.2 KiB
libaom: ∅ → 3.9.0, +8047.8 KiB
libdeflate: ∅ → 1.20, +427.0 KiB
libtiff: ∅ → 4.6.0, +655.9 KiB
libvmaf: ∅ → 3.0.0, +2665.0 KiB
libwebp: ∅ → 1.4.0, +2559.7 KiB
openjpeg: ∅ → 2.5.2, +1525.1 KiB
zstd: ∅ → 1.5.6, +1158.0 KiB

$ nvd diff $(nix build nixpkgs#ffmpeg-headless^bin --print-out-paths --no-link) $(nix build .#ffmpeg-headless^bin --print-out-paths --no-link)
<<< /nix/store/4n60lnj3zkjpasd4c56bzhpx2m8lc1sx-ffmpeg-headless-6.1.1-bin
>>> /nix/store/884f487w5hac6rs94jq6hq5zqkxdv666-ffmpeg-headless-6.1.1-bin
Added packages:
[A.]  #1  celt        0.11.3
[A.]  #2  giflib      5.2.2
[A.]  #3  lcms2       2.16
[A.]  #4  lerc        4.0.0
[A.]  #5  libaom      3.9.0
[A.]  #6  libdeflate  1.20
[A.]  #7  libtiff     4.6.0
[A.]  #8  libvmaf     3.0.0
[A.]  #9  libwebp     1.4.0 x2
[A.]  #10  openjpeg    2.5.2
[A.]  #11  zstd        1.5.6
Closure size: 66 -> 78 (15 paths added, 3 paths removed, delta +12, disk usage +18.5MiB).
```

Strongly inspired by the forgejo counterpart[1], for the following
reasons:

* The feature is broken with the current module and crashes on
  authentication with the following stacktrace (with a PAM service
  `gitea` added):

      server # Stack trace of thread 1008:
      server # #0  0x00007f3116917dfb __nptl_setxid (libc.so.6 + 0x8ddfb)
      server # #1  0x00007f3116980ae6 setuid (libc.so.6 + 0xf6ae6)
      server # #2  0x00007f30cc80f420 _unix_run_helper_binary (pam_unix.so + 0x5420)
      server # #3  0x00007f30cc8108c9 _unix_verify_password (pam_unix.so + 0x68c9)
      server # #4  0x00007f30cc80e1b5 pam_sm_authenticate (pam_unix.so + 0x41b5)
      server # #5  0x00007f3116a84e5b _pam_dispatch (libpam.so.0 + 0x3e5b)
      server # #6  0x00007f3116a846a3 pam_authenticate (libpam.so.0 + 0x36a3)
      server # #7  0x00000000029b1e7a n/a (.gitea-wrapped + 0x25b1e7a)
      server # #8  0x000000000047c7e4 n/a (.gitea-wrapped + 0x7c7e4)
      server # ELF object binary architecture: AMD x86-64
      server #
      server # [   42.420827] gitea[897]: pam_unix(gitea:auth): unix_chkpwd abnormal exit: 159
      server # [   42.423142] gitea[897]: pam_unix(gitea:auth): authentication failure; logname= uid=998 euid=998 tty= ruser= rhost=  user=snenskek

  It only worked after turning off multiple sandbox settings and adding
  `shadow` as supplementary group to `gitea.service`.

  I'm not willing to maintain additional multiple sandbox settings for
  different features, especially given that it was probably not used for
  quite a long time:

  * There was no PR or bugreport about sandboxing issues related to
    PAM.

  * Ever since the module exists, it used the user `gitea`, i.e. it had
    never read-access to `/etc/shadow`.

* Upstream has it disabled by default[2].

If somebody really needs it, it can still be brought back by an overlay
updating `tags` accordingly and modifying the systemd service config.

[1] 07641a9
[2] https://docs.gitea.com/usage/authentication#pam-pluggable-authentication-module

nixosTests.cryptpad started failing recently.

Investigating the issue shows that seccomp has become problematic during
the init phase, (e.g. this can be reproduced by removing the customize
directory in /var/lib/cryptpad):

machine # [   10.774365] systemd-coredump[864]: Process 756 (node) of user 65513 dumped core.
machine #
machine # Module libgcc_s.so.1 without build-id.
machine # Module libstdc++.so.6 without build-id.
machine # Module libicudata.so.74 without build-id.
machine # Module libicuuc.so.74 without build-id.
machine # Module libicui18n.so.74 without build-id.
machine # Module libz.so.1 without build-id.
machine # Module node without build-id.
machine # Stack trace of thread 756:
machine # #0  0x00007ff951974dcb fchown (libc.so.6 + 0x107dcb)
machine # #1  0x00007ff95490d0c0 uv__fs_copyfile (libuv.so.1 + 0x150c0)
machine # #2  0x00007ff95490d89a uv__fs_work (libuv.so.1 + 0x1589a)
machine # #3  0x00007ff954910c76 uv_fs_copyfile (libuv.so.1 + 0x18c76)
machine # #4  0x0000000000eb8a39 _ZN4node2fsL8CopyFileERKN2v820FunctionCallbackInfoINS1_5ValueEEE (node + 0xab8a39)
machine # #5  0x0000000001cda5e2 Builtins_CallApiCallbackGeneric (node + 0x18da5e2)
[...]
machine # [   10.877468] cryptpad[685]: /nix/store/h4yhhxpfm03c5rgz91q7jrvknh596ly2-cryptpad-2024.12.0/bin/cryptpad: line 3:   756 Bad system call         (core dumped) "/nix/store/fkyp1bm5gll9adnfcj92snyym524mdrj-nodejs-22.11.0/bin/node" "/nix/store/h4yhhxpfm03c5rgz91q7jrvknh596ly2-cryptpad-2024.12.0/lib/node_modules/cryptpad/scripts/build.js"

nodejs 20.18 rightly did not require chown when the source and
destination are the same owner (heck, the script does not run as
root so even if it is not blocked there is no way it'd work with a
different owner...)

For now just allow chown calls again, this is not worth wasting more
time.

Fixes NixOS#370717

fluent-bit 3.2.7, 3.2.8 and 3.2.9 are segfaulting when
used in combination with the systemd input. Lets
revert to 3.2.6 for now.

Upstream bug: fluent/fluent-bit#10139

Note that fluent-bit-3.2.7 fixes two high CVEs which we are now
reintroducing. However they are only exploitable if you are
using the OpenTelemetry input or the Prometheus Remote Write input.

OpenTelemetry input: [CVE-2024-50609](https://nvd.nist.gov/vuln/detail/CVE-2024-50609)
Prometheus Remote Write input: [CVE-2024-50608](https://nvd.nist.gov/vuln/detail/CVE-2024-50608)

The problem is as follows:

3.2.7 started vendoring a copy of `libzstd` in tree and statically
linking against it. Also, the fluent-bit binary exports the symbols
of static libraries it links against.

This is a problem because `libzstd` gets `dlopen()`ed by `libsystemd`
when enumerating the journal (as journal logs are zstd compressed). and `libzstd` in Nixpkgs is built
with `-DZSTD_LEGACY_SUPPORT=0` which causes `struct ZSTD_DCtx` to be 16
bytes smaller than without this flag https://github.com/facebook/zstd/blob/dev/lib/decompress/zstd_decompress_internal.h#L183-L187

`libsystemd` calls [`sym_ZSTD_createDCtx()`](https://github.com/systemd/systemd/blob/1e79a2923364b65fc9f347884dd5b9b2087f6e32/src/basic/compress.c#L480)
which calls the function pointer returned by `dlsym()` which is calling into
the `libzstd` that comes with `nixpkgs` and thus allocates a struct that is 16 bytes smaller.

Later then `sym_ZSTD_freeDCtx()` is called. However because fluent-bit
has `zstd` in its global symbol table, any functions that `sym_ZSTD_freeDCtx()`
calls will be calls to the functions in the vendored fluent-bit version of the library
which expects the larger struct. This then causes enough heap corruption to cause
a segfault.

E.g. the subsequent calls to `ZSTD_clearDict(dctx)` and `ZSTD_customFree(dctx->inBuff)`
in https://github.com/facebook/zstd/blob/dev/lib/decompress/zstd_decompress.c#L324
will be working on a struct that is 16 bytes smaller than the one that was allocated
by `libsystemd` and will cause a segfault at some point and thus are probably modifying
pieces of memory that they shouldn't

	(gdb) bt
	#0  0x00007f10e7e9916c in __pthread_kill_implementation () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#1  0x00007f10e7e40e86 in raise () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#2  0x00007f10e7e2893a in abort () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#3  0x000000000046a938 in flb_signal_handler ()
	#4  <signal handler called>
	#5  0x00007f10e7ea42b7 in unlink_chunk.isra () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#6  0x00007f10e7ea45cd in _int_free_create_chunk () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#7  0x00007f10e7ea5a1c in _int_free_merge_chunk () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#8  0x00007f10e7ea5dc9 in _int_free () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#9  0x00007f10e7ea8613 in free () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#10 0x00007f10e80ad3b5 in ZSTD_freeDCtx () from /nix/store/wy0slah6yvchgra8nhp6vgrqa6ay72cq-zstd-1.5.6/lib/libzstd.so.1
	#11 0x00007f10e8c90f6b in decompress_blob_zstd () from /nix/store/b2cfj7yk3wfg1jdwjzim7306hvsc5gnl-systemd-257.3/lib/libsystemd.so.0
	#12 0x00007f10e8bf0efe in journal_file_data_payload () from /nix/store/b2cfj7yk3wfg1jdwjzim7306hvsc5gnl-systemd-257.3/lib/libsystemd.so.0
	#13 0x00007f10e8c00f74 in sd_journal_enumerate_data () from /nix/store/b2cfj7yk3wfg1jdwjzim7306hvsc5gnl-systemd-257.3/lib/libsystemd.so.0
	#14 0x00000000004eae2f in in_systemd_collect ()
	#15 0x00000000004eb5a0 in in_systemd_collect_archive ()
	#16 0x000000000047aa18 in flb_input_collector_fd ()
	#17 0x0000000000495223 in flb_engine_start ()
	#18 0x000000000046f304 in flb_lib_worker ()
	#19 0x00007f10e7e972e3 in start_thread () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	#20 0x00007f10e7f1b2fc in __clone3 () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6

Reverts 7310ab3
Reverts 4fbc6cf

CMakeLists: add find_package YAMLCPP

On my systems there is a path in the fonts list that looks like
/nix/store/<hash>-<font>.ttf. The previous code assumed that every font path
contains at least one more slash which isn't the case here.

The code matched on the / after the $out path of the derivation:

/nix/store/<hash>-<name>/share/X11/fonts/...
                        ^

That slash only exists if the font lives in a subdirectory, which
isn't strictly required. In my case I've a few font files that are
located at /nix/store/<hash>-<name>.ttf and are assembled into a
folder as symlinks. (See example below)

Any attempt at calling realpath(3) on them (potentially recursively)
will result in the single file entry in the store being returned.

Whenever that font is being loaded flatpak segfaulted like so:

```
(gdb) bt
#0  0x00005583b3673763 in get_nix_closure ()
#1  0x00005583b3673711 in get_nix_closure ()
#2  0x00005583b36737dc in add_nix_store_symlink_targets ()
#3  0x00005583b36749a8 in add_font_path_args ()
#4  0x00005583b367b3f4 in flatpak_run_app ()
#5  0x00005583b35e4f08 in flatpak_builtin_run ()
#6  0x00005583b35a5970 in main ()
(gdb) x/5i $rip
=> 0x5583b3673763 <get_nix_closure+307>:	movb   $0x0,(%rax)
   0x5583b3673766 <get_nix_closure+310>:	call   0x5583b35a46a0 <g_hash_table_add@plt>
   0x5583b367376b <get_nix_closure+315>:	jmp    0x5583b3673699 <get_nix_closure+105>
   0x5583b3673770 <get_nix_closure+320>:	xor    %edi,%edi
   0x5583b3673772 <get_nix_closure+322>:	lea    0x30475(%rip),%rsi        # 0x5583b36a3bee
(gdb) x/s $rsi
0x5583e92d2180:	"/nix/store/<hash>-A.ttf"
(gdb) print $rax
$1 = 0
```

RAX pointing to 0 here corresponds to the pointer `p` in the source
code. So the code attempted to dereference a null pointer as no further
slash (the return value of strchr(3)) could be found.

The modification in this commit ensures that we check for `p != 0` so
we don't run into this situation again.

Adding the path to the closure will still be done, even if no further
slash can be found, as that still points to a valid store path.

----

Nix code for custom fonts:

```nix
let
  files = [
    ./A.ttf
    ./B.ttf
  ];
in
runCommandNoCC "custom-fonts" {} ''
  mkdir -p $out/share/fonts/truetype
  cd $out/share/fonts/truetype
  ${ lib.concatStringsSep "\n" (map (file: "ln -s ${file} ${baseNameOf file}") files)}
''
```