l2tp: Avoid sending CDN with reserved result value

Update call to l2tp_send_CDN() to avoid using the 0 result code
(reserved by RFC 2661).
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>

accel-pppd/ctrl/l2tp/l2tp.c

@@ -1196,8 +1196,8 @@ static int l2tp_recv_ICRQ(struct l2tp_conn_t *conn, struct l2tp_packet_t *pack)
 	if (!assigned_sid) {
 		if (conf_verbose)
 			log_ppp_warn("l2tp: ICRQ: no Assigned-Session-ID attribute present in message\n");
-		res = 0;
-		err = 0;
+		res = 2;
+		err = 6;
 		goto out_reject;
 	}