xwm: WM_TRANSIENT_FOR should not point to override-redirect window

The override-redirect window will not be assigned a shell_surface
object. If it is used as a parent window, it will cause a crash
when calling the set_parent function.

The EWMH specification does not describe the behavior of an
override-redirect window as a parent window, so we should ignore
this case.

Signed-off-by: Liu, Kai1 <kai1.liu@intel.com>