rigidbody: Fix regression in background scene fix

Since BKE_object_where_is_calc() might be called outside of
scene_update_tagged_recursive(), we need to fall back to the scene's
rigid body world.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54617 954f8c5b-7b00-dc11-b283-0030488c597c