bug#23809: 24.5; Positive argument to 'posn-at-point' throws erros as being negative

[Eli Zaretskii]

> Date: Tue, 21 Jun 2016 15:07:57 +0200
> From: martin rudalics <rudalics@gmx.at>
> CC: 23809@debbugs.gnu.org
> 
>  > Sounds like a duplicate of bug #21732.
>  >
>  >> With these steps, I get this backtrace:
>  >>
>  >>    Debugger entered--Lisp error: (wrong-type-argument wholenump -14)
>  >>      posn-at-point(619194 #)
> 
> Without further forensics we could "fix" both bugs using the attached
> patch.  WDYT?

I don't like sweeping problems under the carpet.  We should understand
how come we get negative pixel coordinates in these cases, it's most
likely some bug in pos_visible_p or somesuch.