network: fix use-after-free

If the lifetime of the route is already expired, do not try to
configure it.

Fixes a use-after-free, as the Request object is already freed, thus, we
cannot use Route or Link stored in Request object.

src/network/networkd-route.c

@@ -1382,6 +1382,8 @@ static int route_process_request(Request *req, Link *link, Route *route) {
                         }
                 else
                         route_cancel_requesting(route);
+
+                return 1;
         }
 
         r = route_configure(route, sec, link, req);