Add to sbappendstream_locked() a check against NULL mbuf, like it is done
in sbappend_locked() and sbappendrecord_locked().

This is a quick fix to the panic introduced by r274712.

A proper solution should be to make sosend_generic() avoid calling
pru_send() with NULL mbuf for the protocols that do not understand
control messages. Those protocols that understand control messages,
should be able to receive NULL mbuf, if control is non-NULL.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275955 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f