Fix kernel panic while trying to read multicast stream.

When VIMAGE is enabled make sure the "m_pkthdr.rcvif" pointer is set
for all mbufs being input by the IGMP/MLD6 code. Else there will be a
NULL-pointer dereference in the netisr code when trying to set the
VNET based on the incoming mbuf. Add an assert to catch this when
queueing mbufs on a netisr to make debugging of similar cases easier.

Found by: Vladislav V. Prodan
PR: 244002
Reviewed by: bz@
MFC after: 1 week
Sponsored by: Mellanox Technologies