tcp: Fix order of tests in tcp_retransmit_skb()

tcp_write_queue_next() must only be made if we know that
tcp_skb_is_last() evaluates to false.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -1932,8 +1932,8 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
 	/* Collapse two adjacent packets if worthwhile and we can. */
 	if (!(TCP_SKB_CB(skb)->flags & TCPCB_FLAG_SYN) &&
 	    (skb->len < (cur_mss >> 1)) &&
-	    (tcp_write_queue_next(sk, skb) != tcp_send_head(sk)) &&
 	    (!tcp_skb_is_last(sk, skb)) &&
+	    (tcp_write_queue_next(sk, skb) != tcp_send_head(sk)) &&
 	    (skb_shinfo(skb)->nr_frags == 0 &&
 	     skb_shinfo(tcp_write_queue_next(sk, skb))->nr_frags == 0) &&
 	    (tcp_skb_pcount(skb) == 1 &&