bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> Date: Mon, 01 Aug 2022 12:51:47 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: gerd.moellmann@gmail.com, 56682@debbugs.gnu.org, larsi@gnus.org, 
>     monnier@iro.umontreal.ca
> 
> > Which means, for example, that tests like this:
> >
> >  if (current_buffer->long_line_optimizations_p)
> >    {
> >      if (!it->narrowed_begv  <<<<<<<<<<<<<<<<<<<<<<<<<<
> >         || ((pos.charpos < it->narrowed_begv || pos.charpos > 
> > it->narrowed_zv)
> >             && (!redisplaying_p || it->line_wrap == TRUNCATE)))
> >
> > are not necessarily reliable, because we never initialize narrowed_begv 
> > to zero, AFAICT.  Right?
> >
> 
> Indeed.  That wasn't a problem with the previous code in which 
> narrowed_begv was unconditionally assigned.  Now it is.  I think the 
> following change should be enough to fix this.  Agreed?

Yes, of course.