Undefined behavior in AutoFile::write (gcc only) #28761

$ CC=gcc CXX=g++ ./configure --with-sanitizers=undefined && make
$ export UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=1:report_error_type=1
$ FUZZ=utxo_snapshot ./src/test/fuzz/fuzz ../qa-assets/fuzz_seed_corpus/utxo_snapshot/
streams.cpp:48:24: runtime error: null pointer passed as argument 1, which is declared to never be null
    [#0](/bitcoin-bitcoin/0/) 0x5634737bcdc0 in AutoFile::write(Span<std::byte const>) src/streams.cpp:48
    [#1](/bitcoin-bitcoin/1/) 0x5634749e553b in void Serialize<AutoFile, unsigned char const>(AutoFile&, Span<unsigned char const>) serialize.h:288
    [#2](/bitcoin-bitcoin/2/) 0x5634749e553b in AutoFile& AutoFile::operator<< <Span<unsigned char const> >(Span<unsigned char const> const&) streams.h:527
    [#3](/bitcoin-bitcoin/3/) 0x5634749e553b in utxo_snapshot_fuzz_target test/fuzz/utxo_snapshot.cpp:45
    ...
$ ./src/test/test_bitcoin --run_test="net_tests"
streams.cpp:49:24: runtime error: null pointer passed as argument 1, which is declared to never be null
    [#0](/bitcoin-bitcoin/0/) 0x55613837a3ee in AutoFile::write(Span<std::byte const>) src/streams.cpp:49
    [#1](/bitcoin-bitcoin/1/) 0x55613b9747cc in void Serialize<AutoFile, unsigned char const>(AutoFile&, Span<unsigned char const>) serialize.h:288
    [#2](/bitcoin-bitcoin/2/) 0x55613b9747cc in AutoFile& AutoFile::operator<< <Span<unsigned char const> >(Span<unsigned char const> const&) streams.h:527
    [#3](/bitcoin-bitcoin/3/) 0x55613b9747cc in CaptureMessageToFile src/net.cpp:3885

This is harmless since the spans passed to AutoFile::write will always be of size 0 in this case.

I think this is a "bug" in gcc that should be fixed upstream. See also the related discussion in https://reviews.llvm.org/D86993

Also, a suppression can be added in the meantime, if anyone is using g++