bug#61704: 29.0.60; Crash in get_narrowed_begv

[Gregory Heytings <gregory@heytings.org>]

>> and OTOH the redisplay engine should be able to handle zero window 
>> sizes to avoid crashes the window code cannot prevent.
>
> This is the only reasonable approach under X.
>

I maintain that the bug is (was) most probably not in get_narrowed_*.  I 
just built Emacs with the GTK, Lucid and no toolkits, I disabled 
everything (tool-bar, menu-bar, scroll-bar, fringe-mode 0), and even with 
that, after resizing the Emacs frame until it is literally a single pixel 
on screen, I'm not able to get (under emacs -Q) a window width or a height 
equal to 0 there.  The smallest values I get are width = 2 and height = 1.

That, the fact that this code has been there for a half year without a 
single reported crash (if resizing a window is enough to trigger such a 
crash, that would surely have happened already), and the fact that 
get_narrowed_* is not executed at all in the ediff control frame, means 
that there is a bug somewhere else.