pfil hooks (i.e. firewalls) may pass, modify or free the mbuf passed
to them. (E.g. when rejecting a packet, or when gathering up packets
for reassembly).

If the hook returns PFIL_PASS the mbuf must still be present. Assert
this in pfil_mem_common() and ensure that ipfilter follows this
convention. pf and ipfw already did.
Similarly, if the hook returns PFIL_DROPPED or PFIL_CONSUMED the mbuf
must have been freed (or now be owned by the firewall for further
processing, like packet scheduling or reassembly).

This allows us to remove a few extraneous NULL checks.

Suggested by: tuexen
Sponsored by: Rubicon Communications, LLC ("Netgate")