wildcardmatch: fix heap buffer overflow in setcharset

The code would previous read beyond the end of the pattern string if the
match pattern ends with an open bracket when the default pattern
matching function is used.

Detected by OSS-Fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4161

CVE-2017-8817

Bug: https://curl.haxx.se/docs/adv_2017-ae72.html