cxgbe/t4_tom: Do not attempt to look up entries in the TCB history if

it hasn't been initialized.

This fixes a bug in r346570 that could cause a panic when servicing
TCP_INFO for offloaded connections.

MFC after:	3 days
Sponsored by:	Chelsio Communications

sys/dev/cxgbe/tom/t4_tom.c

@@ -537,6 +537,9 @@ lookup_tcb_histent(struct adapter *sc, u_int tid, bool addrem)
 
 	MPASS(tid < sc->tids.ntids);
 
+	if (td->tcb_history == NULL)
+		return (NULL);
+
 	if (addrem)
 		rw_wlock(&td->tcb_history_lock);
 	else