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