Fix handling of empty SCM_RIGHTS messages.

As unp_internalize() processes the input control messages, it builds
an output mbuf chain containing the internalized representations of
those messages. In one special case, that of an empty SCM_RIGHTS
message, the message is simply discarded. However, the loop which
appends mbufs to the output chain assumed that each iteration would
produce an mbuf, resulting in a null pointer dereference if an empty
SCM_RIGHTS message was followed by a non-empty message.

Fix this by advancing the output mbuf chain tail pointer only if an
internalized control message was produced.

Reported by: syzbot+1b5cced0f7fad26ae382@syzkaller.appspotmail.com
MFC after: 1 week
Sponsored by: The FreeBSD Foundation