When iterating on commit 3c1554e7f2 ("lib: Add new app2_class field
for classifying applications") in early versions I was accidentally
passing NULL to match(). Unexpectedly this returned 1 (ie. matching).
This happened because we did not correctly handle other errors apart
from PCRE2_ERROR_NOMATCH from pcre2_match.
Emit a debug message when this happens, and return no match.
To test this I ran these commands, and there was no output:
$ make && LIBGUESTFS_DEBUG=1 LIBGUESTFS_TRACE=1 make check
$ find -name '*.log' | xargs grep 'match.*unexpected error'
28bc6ca6b9