Clean up wp_widget_rss_output():

* Fix appending [&hellip;] to $summary.
* Use wp_trim_words() instead of wp_html_excerpt().
* Trim $title before checking if it's empty. props UmeshSingla.
* Use correct escaping function for $title.

fixes #28356.

git-svn-id: https://develop.svn.wordpress.org/trunk@28586 602fd350-edb4-49c9-b593-d223f7449a82