bug#56393: Actually fix the long lines display bug

[Gregory Heytings]

> > >  emacs -Q
> > >  C-x C-f src/xdisp.c RET
> > >  M->
> > >  C-u 37100 C-p
> > >
> > > This signals an error "Beginning of buffer".
> >
> > Nice.  That's a recipe you concocted knowing the implementation 
> > details, didn't you?  😉
>
> Of course ;-)

Then here's another recipe with long lines detection enabled:

emacs -Q
C-x C-f src/xdisp.c RET
C-n
C-x i .../dictionary.json RET
M->
C-u 37100 C-p

😉