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'
Run this command across the source:
perl -pi.bak -e 's/(20[012][0-9])-20[12][012]/$1-2023/g' `git ls-files`
and remove changes to po{,-docs}/*.po{,t} (these will be regenerated
later when we run 'make dist').
