Fix crash if the SSH server disconnects.

When ssh_remote_error returns, the ssh2_transport_state has been
freed, so we shouldn't try to reach into it to pop its in_pq.

ssh2transport.c

@@ -347,7 +347,7 @@ bool ssh2_common_filter_queue(PacketProtocolLayer *ppl)
                 ((reason > 0 && reason < lenof(ssh2_disconnect_reasons)) ?
                  ssh2_disconnect_reasons[reason] : "unknown"),
                 PTRLEN_PRINTF(msg));
-            pq_pop(ppl->in_pq);
+            /* don't try to pop the queue, because we've been freed! */
             return true;               /* indicate that we've been freed */
 
           case SSH2_MSG_DEBUG: