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>

net/phonet/af_phonet.c

@@ -262,10 +262,9 @@ int pn_skb_send(struct sock *sk, struct sk_buff *skb,
 	else if (phonet_address_lookup(net, daddr) == 0) {
 		dev = phonet_device_get(net);
 		skb->pkt_type = PACKET_LOOPBACK;
-	} else if (pn_sockaddr_get_object(target) == 0) {
+	} else if (dst == 0) {
 		/* Resource routing (small race until phonet_rcv()) */
-		struct sock *sk = pn_find_sock_by_res(net,
-							target->spn_resource);
+		struct sock *sk = pn_find_sock_by_res(net, res);
 		if (sk)	{
 			sock_put(sk);
 			dev = phonet_device_get(net);