fix issue in media library

As I could not set post thumbnail anymore, I looked at the WP_DEBUG_LOG file, which stated:
```[15-Apr-2014 10:30:04 UTC] PHP Notice:  Trying to get property of non-object in /var/www/work/imatra/wp-content/plugins/ajax-thumbnail-rebuild/ajax-thumbnail-rebuild.php on line 66```

Fix was quite simple. You are using `global $post`, but it is not necessary, as the filter passes $post to your function as second parameter.