bug#61704: 29.0.60; Crash in get_narrowed_begv

[Gregory Heytings]

> > How can len possibly be 0 at that point?  It is (in short) 
> > window_body_width (w, WINDOW_BODY_IN_CANONICAL_CHARS) * 
> > window_body_height (w, WINDOW_BODY_IN_CANONICAL_CHARS).  We could add a 
> > condition in get_narrowed_len to return 1 when the result is 0, but it 
> > could be a bug somewhere else (can a window body have a zero width 
> > and/or height?), in which case it would be better to fix the bug there.
>
> I agree that we should understand how this happened (and asked a similar 
> question), but I installed a defensive protection anyway.  It cannot do 
> any harm.

Okay.  Let's hope this will not hide another real bug.

I found a way to get such small windows: set window-min-width, 
window-min-height, window-safe-min-width and window-safe-min-heigth to 0. 
Of course doing that ignores the fact that the docstring of the 
window-safe-min-* variables say that "Anything less might crash Emacs", 
and that their values are 2 (width) and 1 (height).  Which makes me wonder 
why these variables can be changed / exist.