MFC r272797:
Check for mbuf copy failure when there are multiple multicast sockets
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.
git-svn-id: svn://svn.freebsd.org/base/stable/10@274264
ccf9f872-aa2e-dd11-9fc8-
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