bug#61704: 29.0.60; Crash in get_narrowed_begv

[Gregory Heytings]

> The cause is an arithmetic trap in get_narrowed_begv:
>
> return max ((pos / len - 1) * len, BEGV);
>
> where len is 0.  The window was previously being resized, and has a 
> pixel width of 24.

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.