This test crashes the python interpreter in libx265_encode_stream:

```
 #0  0x00007feefe2a7a7e in free () from /nix/store/81mi7m3k3wsiz9rrrg636sx21psj20hc-glibc-2.40-66/lib/libc.so.6
 No symbol table info available.
 #1  0x00007fee98059295 in av_free (ptr=<optimized out>) at libavutil/mem.c:243
 No locals.
 #2  0x00007fee98059352 in av_freep (arg=<optimized out>) at libavutil/mem.c:253
        val = <optimized out>
 #3  0x00007fee997a4713 in libx265_encode_frame (avctx=<optimized out>, pkt=<optimized out>, pic=<optimized out>,
    got_packet=0x7ffe35c7e3f4) at libavcodec/libx265.c:821
    [...]
 #4  0x00007fee99526ff6 in ff_encode_encode_cb (avctx=avctx@entry=0x16d1f00, avpkt=avpkt@entry=0x16ad440, frame=0x16f5e80, got_packet=got_packet@entry=0x7ffe35c7e3f4) at libavcodec/encode.c:254
        codec = 0x7fee9a25e5c0 <ff_libx265_encoder>
 [...]
```

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
	NixOS#9  0x00007f10e7ea8613 in free () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	NixOS#10 0x00007f10e80ad3b5 in ZSTD_freeDCtx () from /nix/store/wy0slah6yvchgra8nhp6vgrqa6ay72cq-zstd-1.5.6/lib/libzstd.so.1
	NixOS#11 0x00007f10e8c90f6b in decompress_blob_zstd () from /nix/store/b2cfj7yk3wfg1jdwjzim7306hvsc5gnl-systemd-257.3/lib/libsystemd.so.0
	NixOS#12 0x00007f10e8bf0efe in journal_file_data_payload () from /nix/store/b2cfj7yk3wfg1jdwjzim7306hvsc5gnl-systemd-257.3/lib/libsystemd.so.0
	NixOS#13 0x00007f10e8c00f74 in sd_journal_enumerate_data () from /nix/store/b2cfj7yk3wfg1jdwjzim7306hvsc5gnl-systemd-257.3/lib/libsystemd.so.0
	NixOS#14 0x00000000004eae2f in in_systemd_collect ()
	NixOS#15 0x00000000004eb5a0 in in_systemd_collect_archive ()
	NixOS#16 0x000000000047aa18 in flb_input_collector_fd ()
	NixOS#17 0x0000000000495223 in flb_engine_start ()
	NixOS#18 0x000000000046f304 in flb_lib_worker ()
	NixOS#19 0x00007f10e7e972e3 in start_thread () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6
	NixOS#20 0x00007f10e7f1b2fc in __clone3 () from /nix/store/rmy663w9p7xb202rcln4jjzmvivznmz8-glibc-2.40-66/lib/libc.so.6

Reverts 7310ab3
Reverts 4fbc6cf

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)}
''
```