CLEANUP: regex: pre-initialize error variable in regex_comp() to calm analysis

CLEANUP: regex: pre-initialize error variable in regex_comp() to calm analysis

In regex_comp(), the error variable is either a const char* (USE_PCRE)
or a a uchar[] (USE_PCRE2), and navigating through the ifdefs is quite a
mess, making it hard to figure if it's always properly initialized when
printing an error message. Let's just preset it to NULL to clarify what
comes from where.