bug#31373: 26.1; frame-list-z-order segfaults on initial daemon frame

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Mon, 7 May 2018 14:08:19 -0400
> Cc: martin rudalics <rudalics@gmx.at>, 31373@debbugs.gnu.org
> 
> >> Thank you.  Eli, I think Noam should push this to 26.1, if still
> >> possible.
> >
> > Why?  The workaround is very simple: don't call that function in a
> > daemon session that has no GUI frames.
> 
> Ah, I think you missed that the segfault happens in a daemon session
> that *does* have GUI frames.

Ugh! why did no one bump into this until now??  We had this
function for a year!

OK, let's push it to emacs-26, and thanks...

Btw, is it reasonable for this function to return nil on TTY frames?
One frame is definitely displayed "on top".