regexp.t: Skip tests that are supposed to

This reorders some if elsif ... blocks so that skip is tested for and
done before actually trying the test.  This only affected tests which
were supposed to generate compiler errors.
regcomp.c: Avoid recalculating a value

This value was calculated and saved in 'len' a couple of lines above.
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.
t/re/re_tests: Remove unused variable initializations

These lines all fail to compile, so matching doesn't happen, so it
doesn't matter at all what the target string to be matched against
is set to.  It is misleading to put apparently meaningful stuff in that
string.
t/re/re_tests: Remove duplicate test

This is identical to the test two lines above.