tcp: fix crash in tcp_xmit_retransmit_queue

It can happen that there are no packets in queue while calling
tcp_xmit_retransmit_queue(). tcp_write_queue_head() then returns
NULL and that gets deref'ed to get sacked into a local var.

There is no work to do if no packets are outstanding so we just
exit early.

This oops was introduced by 08ebd172 (tcp: remove tp->lost_out
guard to make joining diff nicer).
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Reported-by: NLennart Schulte <lennart.schulte@nets.rwth-aachen.de>
Tested-by: NLennart Schulte <lennart.schulte@nets.rwth-aachen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -2208,6 +2208,9 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
 	int mib_idx;
 	int fwd_rexmitting = 0;
 
+	if (!tp->packets_out)
+		return;
+
 	if (!tp->lost_out)
 		tp->retransmit_high = tp->snd_una;