Fix for do_versions bug with node groups: The special case of direct input-to-output connections was not handled correctly. In this case both the tonode and fromnode pointers in old node groups are NULL.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56611 954f8c5b-7b00-dc11-b283-0030488c597c