ttm_bo_unref unconditionally calls kref_put on it's argument, so the
thing can't be NULL without already causing Oopses.
Spotted by coverity.
Reviewed-by: David Herrmann <dh.herrmann@gmail.com> Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>