Phonet: fix NULL dereference on TX path with implicit source

The previous Phonet patch series introduced per-socket implicit
destination (i.e. connect()). In that case, the destination
socket address is NULL in the transmit function.
However commit a8059512
("Phonet: implement per-socket destination/peer address")
is incomplete and would trigger a NULL dereference.
(Fortunately, the code is not in released kernel, and in fact
 currently not reachable.)

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>