[sanitizer] Always initialize the regex in the regcomp() interceptor

When regcomp() fails, the same regex_t* should be passed to regerror()
for potentially better error messages.  But doing that with msan would
report a use-of-uninitialized-value.

Fixes https://github.com/google/sanitizers/issues/1496

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D120591