Draggable: $.ui.ddmanager.current release on draggable _destroy

Fixes: #10667
Is it ever the case that `current` exists but `current.element` does not?

This still doesn't conform to the style guide, but we can fix that when we merge. For reference:
* Use `element[ 0 ]` instead of `element.get( 0 )`; there's never a reason to use `.get()`.
* We use liberal spacing, which you're missing.
* The indentation for the second line of the conditional is wrong.