Add NULL pointer check to nat64_output().

It is possible, that a processed packet was originated by local host,
in this case m->m_pkthdr.rcvif is NULL. Check and set it to V_loif to
avoid NULL pointer dereference in IP input code, since it is expected
that packet has valid receiving interface when netisr processes it.

Obtained from: Yandex LLC
MFC after: 1 week
Sponsored by: Yandex LLC