coverity: avoid deref of explicitly NULL variable on error path

src/lxc/conf.c

@@ -4008,7 +4008,8 @@ static struct lxc_list *idmap_add_id(struct lxc_conf *conf, uid_t uid)
 
 err:
 	ERROR("Out of memory building a new uid map");
-	lxc_free_idmap(new);
+	if (new)
+		lxc_free_idmap(new);
 	free(new);
 	return NULL;
 }