After r338257 is was possible to trigger a KASSERT() in ud6_output()

After r338257 is was possible to trigger a KASSERT() in ud6_output()
using an application trying to use a v4mapped destination address on a
kernel without INET support or on a v6only socket.
Catch this case and prevent the packet from going anywhere;
else, without the KASSERT() armed, a v4mapped destination
address might go out on the wire or other undefined behaviour
might happen, while with the KASSERT() we panic.

PR:		231728
Reported by:	Jeremy Faulkner (gldisater gmail.com)
Approved by:	re (kib)