regcomp.c: Add some asserts()

regcomp.c: Add some asserts()

Now that inversion lists are their own scalar types, we can verify that
the parameters to their manipulation functions are indeed inversion
lists.  This adds such assertions to the bottom level code that deals
with the bare metal of the scalars.  Functions that call these (even if
only in other asserts) didn't have asserts added to them, as they call
these anyway.