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

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Date: Sun, 06 May 2018 12:28:16 -0400
> 
> >> +      if (XFRAME (frame)->output_method == output_x_window
> 
> Also, I noticed that there is FRAME_X_P for this.  I can't find any more
> obvious instances of missing this check, although it's hard to tell in
> many cases if the frame is really guaranteed to have output_x_window has
> the output_method.

Don't we usually signal an error in such cases?  Through
check_x_display_info or somesuch?