regex: fix uninitialized memory access

I introduced this bug into gnulib in commit
8335a4d6c7b4448cd0bcb6d0bebf1d456bcfdb17 dated 2006-04-10;
eventually it was merged into glibc.  The bug was found by
project-repo <bugs@feusi.co> and reported here:
https://lists.gnu.org/r/sed-devel/2018-08/msg00017.html
Diagnosis and draft fix reported by Assaf Gordon here:
https://lists.gnu.org/r/bug-gnulib/2018-08/msg00071.html
https://lists.gnu.org/r/bug-gnulib/2018-08/msg00142.html
* posix/regex_internal.c (build_wcs_upper_buffer):
Fix bug when mbrtowc returns 0.

(cherry picked from commit bc680b33)