Avoid crash at end of list.

diff --git a/windows/winpos.c b/windows/winpos.c
index 8456478..f164c33 100644
--- a/windows/winpos.c
+++ b/windows/winpos.c
@@ -2195,7 +2195,7 @@
 
 	if( hwndInsertAfter != HWND_TOP )
 	{
-	    while( w != wndPtr->owner )
+	    while( w && w != wndPtr->owner )
 	    {
 		if (w != wndPtr) hwndLocalPrev = w->hwndSelf;
 		if( hwndLocalPrev == hwndInsertAfter ) break;