Bug fix #34534

Fix for 2.66a

The new Copy/Paste objects feature could hang in eternal loop.
Only happens for objects that refer to another scene via linkage.

This fix then crashed Blender, needed to add a NULL check for screens.



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55051 954f8c5b-7b00-dc11-b283-0030488c597c