regcomp.c: Fix a panic

regcomp.c: Fix a panic

I haven't figured out a way to get this to panic with existing code
(hence no tests), but chanes I'm working on triggered it.

An inversion list has elements that indicate a range that matches code
points in the list; and elements for ranges that do not match code
points in the list.  This code was assuming that the final range in the
inversion list was in the list; it might not be, and so an extra test is
needed.