_wp_menu_output() speed up and clean up. Props dragoonis. Fixes #13662.

Attempt file_exists() last and cache an array index lookup for speed up. Whitespace and appropriate parentheses for readability.


git-svn-id: https://core.svn.wordpress.org/trunk@18792 1a063a9b-81f0-0310-95a4-ce76da25c4cd