Minor js bug fix - was causing Media Browser button to stop working. 

Fixes Uncaught TypeError: Cannot read property 'length' of undefined javascript error that can cause the Media Browser button (among other js-based things) to stop working.

This particular issue may be specific to my installation but the fix shouldn't hurt anyone else.

Tested on new 1.4.0 version and working. 

There's only the master branch on here now so I guess that's where this should be submitted to?

This fix is also mentioned on wordpress support forum: http://wordpress.org/support/topic/add-media-button-not-working-includes-fix