bug#6400: I want to stay with a sibling window

[martin rudalics]

> There is a bug in the patch. I assumed that if "sib" was non-nil then
> it would be a buffer window. That is not the case so a check of that
> must be made. The most simple is to add another check with
> WINDOW_LIVE_P.

Indeed.  BTW note that the entire job of finding swindow was already
geared towards making the deletion of internal windows possible.

martin