tcp: fix crash in tcp_xmit_retransmit_queue

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 08ebd1721ab8fd (tcp: remove tp->lost_out
guard to make joining diff nicer).

Signed-off-by: Ilpo Järvinen <[email protected]>
Reported-by: Lennart Schulte <[email protected]>
Tested-by: Lennart Schulte <[email protected]>
Signed-off-by: David S. Miller <[email protected]>