cgroup-util: assert on attribute for cg_{get,set}_attribute()

It makes zero sense to ever perform IO operations on cgroup dirs.

src/basic/cgroup-util.c

@@ -1594,6 +1594,8 @@ int cg_set_attribute(const char *controller, const char *path, const char *attri
         _cleanup_free_ char *p = NULL;
         int r;
 
+        assert(attribute);
+
         r = cg_get_path(controller, path, attribute, &p);
         if (r < 0)
                 return r;
@@ -1605,6 +1607,8 @@ int cg_get_attribute(const char *controller, const char *path, const char *attri
         _cleanup_free_ char *p = NULL;
         int r;
 
+        assert(attribute);
+
         r = cg_get_path(controller, path, attribute, &p);
         if (r < 0)
                 return r;