Ensure the validity of hOldFocus before calling WIN_GetTopParent.

windows/winpos.c

@@ -1855,7 +1855,7 @@ BOOL WINPOS_SetActiveWindow( HWND hWnd, BOOL fMouse, BOOL fChangeFocus)
         {
             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 );