regcomp.c: Don't read uninitialized data

The blamed commit failed to check whether the data is present before
reading it.

I believe that this creates false positives in te optimizer, so no
actual failures ensued.
regcomp.c: Use minimal struct formal parameter

The static function get_ANYOF_cp_list_for_ssc() takes a struct formal
parameter that is a superset of what it actually uses.  The calls to it
have to cast to that superset.  By setting the parameter to the smallest
structure it uses, we simplify things.