hwdb: reject overlong fnmatch key instead of passing NULL to fnmatch()

hwdb: reject overlong fnmatch key instead of passing NULL to fnmatch()

When the accumulated trie key exceeds the fixed-size line buffer,
linebuf_get() returns NULL. trie_fnmatch_f() passed that NULL straight
into fnmatch() as the pattern, causing a SIGSEGV on a crafted hwdb.bin
(reachable now that recursion is capped rather than overflowing the
stack first). Treat the NULL like the other corruption checks and
return -EBADMSG.

Follow-up for 73fea38cf1344e08213bb10bfc1e1a98382aee78

Fixes https://github.com/systemd/systemd/issues/42376

Co-developed-by: Claude Opus 4.8 <noreply@anthropic.com>