regcomp.c: Prepare for allowing [^\p{...}]

regcomp.c: Prepare for allowing [^\p{...}]

It turns out that this code is buggy, except for the fact that
<nonbitmap> currently can't contain conflicts.  The trouble would have
started when Unicode properties were moved to being looked at at compile
time -- except when the class is to be inverted, so there isn't a
problem.  But in preparation for handling this case, we fix the
potential bugs, as specified in the comments.