rigidbody: Fix [#33971] Bullet Physics crash + patch

BKE_rigidbody_aftertrans_update() can be called before rigid body is
validated so check before updating physics_object.

Thanks to Antony Riakiotakis (psy-fi) for the initial patch.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54062 954f8c5b-7b00-dc11-b283-0030488c597c