bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > Here's one problem caused by using PT in get_narrowed_begv:
> >
> >  emacs -Q
> >  C-x C-f src/xdisp.c RET
> >  C-x 2
> >  M->
> >
> > Now move the mouse pointer over the "other" window, the one which still 
> > shows xdisp.c at BOB, and turn the mouse wheel to scroll the window 
> > towards the end of the buffer.  This segfaults:
> >
> > The problem is that narrowed_begv was computed using PT, which doesn't 
> > correspond to the window's point, and so the logic in 
> > get_visually_first_element backfires.
>
> Yes, I understand and noted that I should use window-point instead of 
> PT.

(That being said, I cannot reproduce that here either.)