youtube_tokens() regenerates thumbnails on every call

In the youtube_tokens() function, there's a test to see if the YouTube thumbnail already exists:

            $file_uri = youtube_build_thumbnail_uri($video_id);
            $file_url = file_create_url($file_uri);
            if (file_exists($file_url) || youtube_get_remote_image($video_id)) {

It believe that it should be $file_uri that's passed to file_exists(), not $file_url.

Because the file_exists() is always failing, the thumbnail image is being recreated on every call.

In the general run of affairs this is fine, and wouldn't be noticed; in our case, however, with a high-traffic site where the tokenized image was being called several times a second, it dramatically increased the load on our server.

I've attached a patch.