bug#43597: 28.0.50; arc-mode.el fails to display a ZIP file

[Eli Zaretskii]

> From: Stephen Berman <stephen.berman@gmx.net>
> Date: Thu, 24 Sep 2020 22:40:15 +0200
> Cc: 43597@debbugs.gnu.org
> 
>   (let ((p (archive-l-e (+ (point) 16) 4))
>         files)
>     (when (= p -1)
>       ;; If the offset of end-of-central-directory is -1, this is a
>       ;; Zip64 extended ZIP file format, and we need to glean the info
>       ;; from Zip64 records instead.
> 
> But the first value returned by archive-l-e for this file is 255, not
> -1, and on each iteration the value increases, so the ZIP64 code in the
> body of the when-clause is never executed.

This code worked when support for ZIP64 was added.

Could it be that the problem is in archive-l-e?  Something related to
bignums, perhaps?  Why does it return 255 instead of -1 in this case?