Bugfix: Cope with NULL pDriverData in X11DRV_WND_GetXWindow.

windows/x11drv/wnd.c

diff --git a/windows/x11drv/wnd.c b/windows/x11drv/wnd.c
index 60e2a16..2e9cac3 100644
--- a/windows/x11drv/wnd.c
+++ b/windows/x11drv/wnd.c
@@ -49,7 +49,7 @@
  */
 Window X11DRV_WND_GetXWindow(WND *wndPtr)
 {
-    return wndPtr ? 
+    return wndPtr && wndPtr->pDriverData ? 
       ((X11DRV_WND_DATA *) wndPtr->pDriverData)->window : 0;
 }