bug#60096: 29.0.60; Crash in format_mode_line_unwind_data

[martin rudalics]

> Basically, set-window-configuration was called in a situation where it
> has only one valid buffer to play with, which is tough for its logic
> and the various other functions it invokes.  The result was a bunch of
> bad windows with nil as their buffer.

If a live window has nil as its buffer, next redisplay will reliably
crash Emacs anyway.  So this

      /* We may have deleted windows above.  Then again, maybe we
         haven't: the functions we call to maybe delete windows can
         decide a window cannot be deleted.  Force recalculation of
         Vwindow_list next time it is needed, to make sure stale
         windows with no buffers don't escape into the wild, which
         will cause crashes elsewhere.  */
      Vwindow_list = Qnil;

should not be needed.  Otherwise we'd have been in serious trouble ever
since Vwindow_list was added.  The earlier call

      delete_all_child_windows (FRAME_ROOT_WINDOW (f));

should have reliably reset Vwindow_list to nil.

martin