regexec.c: Don't skip trying POSIX class if [] has individ char

commit b99851e1941e002dd4816ee6c76fd49bbee1d7f3
Author: Karl Williamson <public@khwilliamson.com>
Date:   Wed Dec 4 10:06:04 2013 -0700

    PATCH: [perl #120675] Unexpected tainting via regex using locale

introduced a bug in which under /li matching a bracketed character class
that contains both characters and POSIX classes would skip matching
against any POSIX class.  The example in the added test illustrates the
problem:

    "h" =~ /[g\w]/li;    # Failed! in C locale