Update DisplayObject.js

Fix: case setBounds(null)  (don't do "this._bounds = new createjs.Rectangle" after "this._bounds = null").
Maybe we need also always create new rectangle, not using old _bounds (or always return cloned rectangle from getBounds())?
In my project I got several bugs related to next code:
var boundsToUseLater = myObj.getBounds(); // save current bounds
// some code (change myObj then call getBounds())
// use boundsToUseLater - properties x/y/widht/height not the same as they were the save moment