There can never be a parent/child relationship between windows if one of
them is invalid. Fixes bug #217.

reactos/lib/user32/windows

diff -u -r1.105 -r1.106
--- window.c	2 Apr 2004 23:54:26 -0000	1.105
+++ window.c	7 Apr 2004 08:25:40 -0000	1.106
@@ -1,4 +1,4 @@

-/* $Id: window.c,v 1.105 2004/04/02 23:54:26 weiden Exp $

+/* $Id: window.c,v 1.106 2004/04/07 08:25:40 gvg Exp $

  *
  * COPYRIGHT:       See COPYING in the top level directory
  * PROJECT:         ReactOS user32.dll

@@ -1018,6 +1018,11 @@

 IsChild(HWND hWndParent,
 	HWND hWnd)
 {

+   if (! IsWindow(hWndParent) || ! IsWindow(hWnd))
+   {
+       return FALSE;
+   }
+

    do 
    {
       hWnd = (HWND)NtUserGetWindowLong(hWnd, GWL_HWNDPARENT, FALSE);