Fixed the dateinput month/year dropdown bug in Firefox 20 (issue #982)

In case of FF20 the el variable (e.target inside the $(document).on("click.d", function(e) {...} ) is [object HTMLDocument] and so the parents.length is 0.
The reason seems to be the monthSelector.emty(), monthSelector.append() stuff inside the setValue() function. Im able to fix it by excluding this case.

I am sure that this is not the smartest way to fix it, but it seems to work.