Revert rev. 1.94. After recent tcp backouts, tcp_close() may return NULL.

Check the return value of tcp_close() being NULL before dereferencing it
in #ifdef TCPDEBUG block.

Reviewed by:	rwatson
Approved by:	re (gnn)

sys/netinet/tcp_timer.c

@@ -240,7 +240,7 @@ tcp_timer_2msl(void *xtp)
        }
 
 #ifdef TCPDEBUG
-	if (tp->t_inpcb->inp_socket->so_options & SO_DEBUG)
+	if (tp != NULL && (tp->t_inpcb->inp_socket->so_options & SO_DEBUG))
 		tcp_trace(TA_USER, ostate, tp, (void *)0, (struct tcphdr *)0,
 			  PRU_SLOWTIMO);
 #endif