xfr: give up if one rrset is larger than whole message (fix of infinite loop)

src/knot/nameserver/xfr.c

@@ -57,6 +57,12 @@ int xfr_process_list(knot_pkt_t *pkt, xfr_put_cb process_item,
 		ret = knot_pkt_put(pkt, 0, &soa_rr, KNOT_PF_NOTRUNC);
 	}
 
+	/* If a rrset is larger than the message,
+	 * fail to avoid infinite loop of empty messages */
+	if (ret == KNOT_ESPACE && pkt->rrset_count < 1) {
+		return KNOT_ENOXFR;
+	}
+
 	/* Update counters. */
 	xfr_stats_add(&xfer->stats, pkt->size);