--- sys/netinet/tcp_subr.c.orig
+++ sys/netinet/tcp_subr.c
@@ -2216,6 +2216,8 @@
 		tcp_respond(tp, mtod(m, void *), th, m, tp->rcv_nxt,
 		    tp->snd_nxt, TH_ACK);
 		tp->last_ack_sent = tp->rcv_nxt;
+	} else {
+		m_freem(m);
 	}
 }