Fix an infinite recursion caused by missing plugin

Plugin routing causes an infinite recursion if the plugin is missing.
You can reproduce it to create a route such as the following

Router::connect('/foo', array('plugin' => 'non_existent_plugin', 'controller' => 'bar', 'action' => 'baz'));

and access /foo.
I've fixed it.
Although it's a programmer error, I hope to have a kind response.