Ensure the validity of hOldFocus before calling WIN_GetTopParent.

windows/winpos.c

diff --git a/windows/winpos.c b/windows/winpos.c
index 121f650..e2f56e4 100644
--- a/windows/winpos.c
+++ b/windows/winpos.c
@@ -1855,7 +1855,7 @@
         {
             HWND hOldFocus = PERQDATA_GetFocusWnd( pNewActiveQueue->pQData );
 
-            if ( WIN_GetTopParent( hOldFocus ) != hwndActive )
+            if ( hOldFocus && WIN_GetTopParent( hOldFocus ) != hwndActive )
                 FOCUS_SwitchFocus( pNewActiveQueue, hOldFocus, 
                                    (wndPtr && (wndPtr->dwStyle & WS_MINIMIZE))?
                                    0 : hwndActive );