fnmatch: insist on escaped bracket to match

A non-escaped bracket ([) is for a character group - as documented. It
will *not* match an individual bracket anymore. Test case 1307 updated
accordingly to match.

Problem detected by OSS-Fuzz, although this fix is probably not a final
fix for the notorious timeout issues.

Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=8525
Closes #2614