mgw_fsm: release call when FSM is not created

While the FSM is created the RAB Assignment Requests is checked and
parsed. In case of failure the context is freed, but the CN is not
informed about the problem. The RAB AssignmentRequest will then most
likely time out. However, lets make sure the call is released by re
requesting an IU Release.

Change-Id: I1904f7e95d86bbcecee14f8721bd4075d0e33ab4
Related: OS#5152

src/osmo-hnbgw/mgw_fsm.c

@@ -719,9 +719,9 @@ int handle_rab_ass_req(struct hnbgw_context_map *map, struct osmo_prim_hdr *oph,
 
 	return 0;
 error:
-	/* TODO: If we fail in this early stage, we should generate an appropriate RAB AssignmentResponse to inform
-	 * the core network about the failure. */
+	/* Cleanup context and make sure that the call is cleared. */
 	mgw_fsm_priv_cleanup(mgw_fsm_priv);
+	tx_release_req(map);
 	return -EINVAL;
 }