bug#14970: crash deleting frames

[martin rudalics]

> 3216              w = XWINDOW (FRAME_SELECTED_WINDOW (f));

My crystal ball tells me that Eli will check whether f is a live frame
in between the following two statements of w32fns.c

          f = x_window_to_frame (dpyinfo, hwnd);
          w = XWINDOW (FRAME_SELECTED_WINDOW (f));

which should considerably reduce the danger that the frame has been
recycled when accessing FRAME_SELECTED_WINDOW.  I'm not sure whether
it's worth the effort having FRAME_SELECTED_WINDOW check that its
argument is a live frame.

martin