bug#7728: 24.0.50; GDB backtrace from abort

[Eli Zaretskii]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: Drew Adams <drew.adams@oracle.com>,  7728@debbugs.gnu.org
> Date: Sat, 25 Dec 2010 11:44:21 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > So it looks like somehow we get into selected_window a value that is
> > not a window.  I'm guessing it was nil in this case.
> 
> There is exactly one place where selected_window can be nil:
> 
> /* Note that selected_window can be nil when this is called from
>    Fset_window_configuration.  */

And Fset_window_configuration->Fselect_window call is indeed on the
call stack.