bug#39662: 28.0.50; Emacs crashes in x_cr_define_fringe_bitmap if (fringe-mode 0)

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Wed, 19 Feb 2020 09:59:21 +0100
> Cc: Dmitry Gutov <dgutov@yandex.ru>, Thibault Polge <thibault@thb.lt>, 
> 39662@debbugs.gnu.org
> 
> If the user turns fringe-mode back on, diff-hl will use the previously 
> created bitmaps, which will be width 1 if
> define-fringe-bitmap decides to use that instead of zero, or will crash if 
> x_cr_define_fringe_bitmap does not
> define the bitmap when width == 0

Since passing zero invokes undefined behavior, producing 1 from that
is fine.  Crashing is best avoided, though.

> Maybe signaling an error is best :-)

That was my original suggestion/tendency, so I obviously agree.

Thanks.