bug#41520: 28.0.50; Crash in character.h due to assertion error

[Eli Zaretskii <eliz@gnu.org>]

> From: Pip Cet <pipcet@gmail.com>
> Date: Mon, 25 May 2020 14:30:55 +0000
> Cc: stefan@marxist.se, 41520@debbugs.gnu.org
> 
> I'm attaching a patch with a few more cases. I don't have a strong
> preference for either FETCH_BYTE or FETCH_CHAR where both will do, but
> we should be consistent.

I'm okay with those additional changes, but let's install them on
master, as they are a cleanup, not a bug.

> > That would require us to maintain both character and byte positions
> > where we use these macros,
> 
> For now, I'd like to introduce them in Emacs proper only where we have
> pairs of variables anyway.

Maybe a new macro that accepts a 'struct text_pos'?

But wouldn't it be strange to see a macro that accepts a struct, but
only uses one member of that struct?

> > Moreover, I think we prefer having assertions in the debug
> > builds rather then silent fixups (and in production eassume compiles
> > into something that doesn't abort).
> 
> I see no way to have assertions

I mean we already have assertions: that's what eassume does in a debug
build.