Crashes with Firefox 45

[gaearon]

We seem to have two (related?) issues with FF 45 affecting different systems in different ways:

• Dragging something non-draggable breaks everything in Firefox 45 react-dnd#313 (comment) (FF45, Windows 10)
• Dragging something non-draggable breaks everything in Firefox 45 react-dnd#313 (comment) (FF45, OS X)

Originally reported by @SoonDead in react-dnd/react-dnd#313, but this repo is the correct place for the bug.

[gaearon]

Should be fixed in react-dnd-html5-backend@2.0.1. Please verify.
(I only tested on OS X.)