As uipc_internalize() processes input control messages, it forms an
output mbuf chain, placed in *controlp.
The switch statement which handles different message types always
appends a new mbuf to the output chain, with one exception: an empty
SCM_RIGHTS message. The code at the end of the loop did not handle this
exception: it always advances controlp along the mbuf chain. Fix this,
and add a regression test.