Fix a fatal error occurs in combination with a scaffold error.

Rendering scaffold_error.ctp causes a fatal error.
To reproduce, create a controller such as the following.
<pre>
class ArticlesController extends AppController {
	public $scaffold;

	public function beforeScaffold() {
		return false;
	}
}
</pre>
I have fixed this issue. (Though scaffolding will be removed in 3.0)