ike-me: Fix safety check to ensure peer ID payload in ME_CONNECT request

The presence of a peer ID is enforced by the message rules for ME_CONNECT,
this just mirrors the safety check also present in `process_r_ms()`.

Fixes: d5cc175833 ("experimental P2P-NAT-T for IKEv2 merged back from branch")

src/libcharon/sa/ikev2/tasks/ike_me.c

@@ -344,6 +344,7 @@ METHOD(task_t, process_r, status_t,
 			{
 				DBG1(DBG_IKE, "received ME_CONNECT without ID_PEER payload"
 					 ", aborting");
+				this->invalid_syntax = TRUE;
 				break;
 			}
 			this->peer_id = id_payload->get_identification(id_payload);