Eliminate use of `extract()` in `wp_widget_rss_output()`. 

Add `'items' => 0` to `$default_args`. When `0`, the value is set to `10` (the fallback). 
Every other default arg has a default value of `0`.

`items` is expected to always be passed to this function.

See #22400.


git-svn-id: https://develop.svn.wordpress.org/trunk@28440 602fd350-edb4-49c9-b593-d223f7449a82