sixlowpan: ignore incoming packets if unconfigured

If there's no IPv6 address configured so far to any interface, 6lowpan
should not try to  handle incoming packets. This can easily lead to
looping packets.
Why test for `< 1` if the return value is either 0 or 1?
I'd rather take the chance to document `is_our_address` than allowing implicit changes in its behavior work.