rest_api: strtoul canot return negative

>>>     CID 307538:  Control flow issues  (NO_EFFECT)
>>>     This less-than-zero comparison of an unsigned value is never true. "map_id < 0UL".

Closes: CID#307538
Change-Id: Ic5019e216dd7b26be0bd988df218a2cb5775e411

src/server/rest_api.c

@@ -410,7 +410,7 @@ static int api_cb_slotmaps_del(const struct _u_request *req, struct _u_response
 		goto err;
 	}
 	map_id = strtoul(slotmap_id_str, NULL, 10);
-	if (map_id < 0) {
+	if (map_id == ULONG_MAX && errno != 0) {
 		status = 400;
 		goto err;
 	}