fix segfault in error handling for mgw_fi == NULL

In mgw_fsm_handle_rab_ass_resp(), a NULL mgw_fi is handled as error,
but the error handling fails to return. The function continues to
dereference mgw_fi. Add missing return.

Related: SYS#5995
Change-Id: I3e98dc3a00145ec1f71c678bbf45debfd4276237

src/osmo-hnbgw/mgw_fsm.c

@@ -767,6 +767,7 @@ int mgw_fsm_handle_rab_ass_resp(struct hnbgw_context_map *map, struct osmo_prim_
 
 		/* Send a release request, to make sure that the MSC is aware of the problem. */
 		tx_release_req(map);
+		return -1;
 	}
 
 	mgw_fsm_priv = map->mgw_fi->priv;