sctp: Make sure we always return valid retransmit path

commit 4951feda0c60d1ef681f1a270afdd617924ab041
    sctp: Do no select unconfirmed transports for retransmissions

added code to make sure that we do not select unconfirmed paths
for data transmission.  This caused a problem when there are only
2 paths, 1 unconfirmed and 1 unreachable.  In that case, the next
retransmit path returned is NULL and that causes a kernel crash.

The solution is to only change retransmit paths if we found one to use.

Reported-by: Frank Schuster <frank.schuster01@web.de>
Signed-off-b: Vlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/associola.c

@@ -1324,7 +1324,8 @@ void sctp_assoc_update_retran_path(struct sctp_association *asoc)
 		}
 	}
 
-	asoc->peer.retran_path = t;
+	if (t)
+		asoc->peer.retran_path = t;
 
 	SCTP_DEBUG_PRINTK_IPADDR("sctp_assoc_update_retran_path:association"
 				 " %p addr: ",