ipfw: Fix segfault in NPTv6 rule parser

If the user specified a prefix length with either the internal or
external prefix, we'd jump to check_prefix where we'd dereference p
which was most likely uninitialized.

Instead, store the various prefix lengths separately and check them
all after the loop.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D50597