Fix NULL pointer deref in ipfw when using dummynet at layer 2.

Fix NULL pointer deref in ipfw when using dummynet at layer 2.
Drop packet if pkg->ifp is NULL, which is the case here.

ref. https://github.com/HardenedBSD/hardenedBSD
commit 4eef3881c64f6e3aa38eebbeaf27a947a5d47dd7

PR 193861 --  DUMMYNET LAYER2: kernel panic

in this case a kernel panic occurs. Hence, when we do not get an interface,
we just drop the packet in question.

PR:		193681
Submitted by:	David Carlier <david.carlier@hardenedbsd.org>
Obtained from:	Hardened BSD
MFC after:	2 weeks
Relnotes:	yes