bug#20663: page.el (forward-page): Avoid skipping pages

[Eli Zaretskii]

> From: Marcin Borkowski <mbork@mbork.pl>
> Cc: ambrevar@gmail.com, 20663@debbugs.gnu.org
> Date: Wed, 13 Apr 2016 19:53:31 +0200
> 
> > Can one of you please explain why the original code misbehaves?
> 
> Quoting from the original report:
> 
> --8<---------------cut here---------------start------------->8---
> > Running `emacs -Q example.txt`:
> >
> >     M-<
> >     C-x n p
> >     M->
> >     M-1 C-x n p
> >
> > This should bring us from page 1 to page 2, but page 3 gets displayed 
> > instead.
> --8<---------------cut here---------------end--------------->8---

Yes, I've read that.  I asked for an explanation of _why_ the code
currently in Emacs misbehaves in this recipe.  Can one of you describe
that?

Thanks.