tcp: Fix queue traversal in tcp_use_frto().

We must check tcp_skb_is_last() before doing a tcp_write_queue_next().
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_input.c

@@ -1746,6 +1746,8 @@ int tcp_use_frto(struct sock *sk)
 		return 0;
 
 	skb = tcp_write_queue_head(sk);
+	if (tcp_skb_is_last(sk, skb))
+		return 1;
 	skb = tcp_write_queue_next(sk, skb);	/* Skips head */
 	tcp_for_write_queue_from(skb, sk) {
 		if (skb == tcp_send_head(sk))