posix: Falling back to non wide mode in case of encoding error [BZ #14185]

Gnulib has added the proposed fix with aed23714d60 (done in 2005), but
recently with a glibc merge with 67306f6 (done in 2020 with sync back)
it has fallback to old semantic to return -1 on in case of failure.

From gnulib developer feedback it was an oversight.  Although the full
fix for BZ #14185 would require to rewrite fnmatch implementation to use
mbrtowc instead of mbsrtowcs on the full input, this mitigate the issue
and it has been used by gnulib for a long time.

This patch also removes the alloca usage on the string convertion to
wide characters before calling the internal function.

Checked on x86_64-linux-gnu.