Remove a KASSERT which is not always true.

In case of the empty queue tp->snd_holes and tcp_sackhole_insert()
failing due to memory shortage, tp->snd_holes will be empty.
This problem was hit when stress tests where performed by pho.

PR:		215513
Reported by:	pho
Tested by:	pho
Sponsored by:	Netflix, Inc.

sys/netinet/tcp_sack.c

@@ -470,9 +470,6 @@ tcp_sack_doack(struct tcpcb *tp, struct tcpopt *to, tcp_seq th_ack)
 		tp->snd_fack = sblkp->end;
 		sack_changed = 1;
 	}
-	/* We must have at least one SACK hole in scoreboard. */
-	KASSERT(!TAILQ_EMPTY(&tp->snd_holes),
-	    ("SACK scoreboard must not be empty"));
 	cur = TAILQ_LAST(&tp->snd_holes, sackhole_head); /* Last SACK hole. */
 	/*
 	 * Since the incoming sack blocks are sorted, we can process them