wp_mime_type_icon PHP notice fixes

[sc0ttkclark]

wp_mime_type_icon ends up sending to the 'wp_mime_type_icon' filter: $icon, $mime, and $post_id

Based on the PHP, it's set in such a way that PHP notices could be generated for variables don't exist.

I've attached my patch that should resolve those potential notices.

[sc0ttkclark]

Oops, missed it by one number, pretend my patch was really 21614.patch :)

[nacin]

Duplicate of #21828.

[sc0ttkclark]

This ticket was duplicated by #21835 (partially), but committed as part of the patch that went in for it. (just noting here for reference)