Make sure settings.plup is defined before trying to attach Plupload to fields

Hi,

I encountered a small issue where Plup would cause a javascript error "TypeError: j is undefined" when editing a node where the content type has two image fields, with only one is set use Plup.
If the user removes the image from the field not using Plup, the attach behavior is called but settings.plup is null and the jQuery.each loop fails.

Attached is a small patch that fixes this.