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

[Gregory Heytings]

> > > 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.

Thanks; now pushed.