coverity: fix memleak in userns_exec_1

src/lxc/conf.c

@@ -4051,6 +4051,7 @@ int userns_exec_1(struct lxc_conf *conf, int (*fn)(void *), void *data)
 
 	ret = lxc_map_ids(idmap, pid);
 	lxc_free_idmap(idmap);
+	free(idmap);
 	if (ret < 0) {
 		ERROR("Error setting up child mappings");
 		goto err;