If this method was called on a key file key which has an empty value,
`len == 0` and this leads to a one-byte under-read off the start of the
key file buffer.
Spotted by linhlhq as #YWH-PGM9867-200. The suggested fix is theirs, and
the unit test is adapted from their report. I added the fuzzing test.
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Fixes: #3930
The files have only been touched by a subset of three people: pdknsk,
Philip Withnall, and Marc-André Lureau. Their copyrights are assigned to
pdknsk, Endless OS Foundation and Red Hat.
The default license for GLib at the time of writing these files was (and
still is) LGPL-2.1-or-later.
`driver.c` came from LLVM and is under a different license:
https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/fuzzer/standalone/StandaloneFuzzTargetMain.c.
That doesn’t affect the license of GLib overall, since it’s only used
for testing during development.
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: #1415
