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: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Reported-by: Lennart Schulte <lennart.schulte@nets.rwth-aachen.de>
Tested-by: Lennart Schulte <lennart.schulte@nets.rwth-aachen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>