Previous commit causes the warning to be triggered, but the suboptimal return
from const statement was added earlier in 517e204.

Warning was causing CI failure https://cirrus-ci.com/task/6159914970644480

Add util::Result Update method to make it possible to change the value of an
existing Result object. This will be useful for functions that can return
multiple error and warning messages, and want to be able to change the result
value while keeping existing messages.

…ainstate

Add util::Result support for returning warning messages and multiple errors,
not just a single error string. This provides a way for functions to report
errors and warnings in a standard way, and simplifies interfaces.

The functionality is unit tested here, and put to use in followup PR
bitcoin#25722

Suggested by Martin Leitner-Ankerl <[email protected]>
bitcoin#25722 (comment)

Co-authored-by: Martin Leitner-Ankerl <[email protected]>

Avoid false positive maybe-uninitialized errors in
00_setup_env_native_fuzz_with_valgrind CI jobs. Similar errors also happen in
00_setup_env_arm and 00_setup_env_win64 jobs.

Problem was pointed out and fix was suggested by maflcko in
bitcoin#25665 (comment)

CI errors look like:
https://github.com/bitcoin/bitcoin/actions/runs/18881332233/job/53885646274?pr=25665

/home/admin/actions-runner/_work/_temp/src/wallet/test/util.cpp: In function ‘wallet::DescriptorScriptPubKeyMan* wallet::CreateDescriptor(CWallet&, const std::string&, bool)’:
/home/admin/actions-runner/_work/_temp/src/wallet/test/util.cpp:213:29: error: ‘*(const std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>*)((char*)&spkm + offsetof(util::Result<std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>, void, util::Messages>,util::Result<std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>, void, util::Messages>::<unnamed>.util::detail::SuccessHolder<std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>, void, util::Messages>::<unnamed>)).std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>::_M_data’ may be used uninitialized [-Werror=maybe-uninitialized]
  213 |     return &spkm.value().get();
      |             ~~~~~~~~~~~~~~~~^~
/home/admin/actions-runner/_work/_temp/src/wallet/test/util.cpp:212:10: note: ‘*(const std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>*)((char*)&spkm + offsetof(util::Result<std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>, void, util::Messages>,util::Result<std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>, void, util::Messages>::<unnamed>.util::detail::SuccessHolder<std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>, void, util::Messages>::<unnamed>)).std::reference_wrapper<wallet::DescriptorScriptPubKeyMan>::_M_data’ was declared here
  212 |     auto spkm = Assert(keystore.AddWalletDescriptor(w_desc, keys,/*label=*/"", /*internal=*/false));
      |          ^~~~

The util::ResultPtr class is a wrapper for util::Result providing syntax sugar
to make it less awkward to use with returned pointers. Instead of needing to be
deferenced twice with **result or (*result)->member, it only needs to be
dereferenced once with *result or result->member. Also when it is used in a
boolean context, instead of evaluating to true when there is no error and the
pointer is null, it evaluates to false so it is straightforward to determine
whether the result points to something.

This PR is only partial a solution to bitcoin#26004 because while it makes it easier
to check for null pointer values, it does not make it impossible to return a
null pointer value inside a successful result. It would be possible to enforce
that successful results always contain non-null pointers by using a not_null
type (bitcoin#24423) in combination with
ResultPtr, though.