Toolbar: Improvements to changes in [62282].

This updates the JavaScript based regular expression to be case-insensitive, which matches the corresponding PHP pattern.

A defensive check has alos been added to avoid a `TypeError` if the admin bar is manipulated in a way that changes the path of the targeted element.

Follow up to [62282].

Props westonruter.
Fixes #65121.

git-svn-id: https://develop.svn.wordpress.org/trunk@62320 602fd350-edb4-49c9-b593-d223f7449a82