[PACKET]: Fix deref before NULL check in packet_release()

Using the automated source checker at coverity.com, they picked up
on some code in packet_release() where a NULL check was done
after dereferencing.  Patch below.
Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -786,11 +786,13 @@ static int packet_sendmsg(struct kiocb *iocb, struct socket *sock,
 static int packet_release(struct socket *sock)
 {
 	struct sock *sk = sock->sk;
-	struct packet_opt *po = pkt_sk(sk);
+	struct packet_opt *po;
 
 	if (!sk)
 		return 0;
 
+	po = pkt_sk(sk);
+
 	write_lock_bh(&packet_sklist_lock);
 	sk_del_node_init(sk);
 	write_unlock_bh(&packet_sklist_lock);