This partitular case is the only path where the mbuf could be NULL.
udp_append() checked for a NULL mbuf only after invoking the tunneling
callback. Our only in tree tunneling callback - SCTP - assumed a non
NULL mbuf, and it is a bit odd to make the callbacks responsible for
checking this condition.
This also reduces the differences between the IPv4 and IPv6 code.
MFC after: 1 month