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

[Stephen Berman]

I have a ZIP file that, when I try to visit it in archive-mode, Emacs
throws a file mode specification error on, specifically
args-out-of-range.  The reason is that the function archive-l-e in
arc-mode.el, which is supposed to return a buffer position in this case,
returns a number which exceeds the buffer size.  And the reason for this
is that archive-l-e builds its return value by iteratively calling (+
(ash result 8) (aref str (- len i))), where `str' is a string extracted
from the end of the ZIP file, which in this case is "\377\377\377\377",
so that the return value increases from 255 (the decimal value of octal
377) to 4294967040 after four iterations, which far exceeds the file
(and hence buffer) size.

Perhaps "\377\377\377\377" is an invalid string at the end of a ZIP file
(I checked other ZIP files I have, which I can visit in archive-mode
with no problem, and they have different strings at the end, consisting
of bytes with smaller decimal values, so that the return value of
archive-l-e does not exceed the file size).  Yet when I call `unzip -l'
on the file in the shell, the contents are displayed, and I also had no
problem unpacking the file with unzip (and AFAICT the content is
undamaged).  So at least unzip can deal with this file.

In case it helps, here is the final byte sequence in the file (I've
replace the control characters and raw bytes by ASCII representations):

PK^E^F\377\377\377\377%^@%^@\377\377\377\377\377\377\377\377^@^@


In GNU Emacs 28.0.50 (build 25, x86_64-pc-linux-gnu, GTK+ Version 3.24.17, 
cairo version 1.17.3)
 of 2020-09-24 built on strobe-jhalfs
Repository revision: 89dd8cd215148da4c6dffc15dc6c35df5122247b
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Linux From Scratch SVN-20200401

Configured using:
 'configure 'CFLAGS=-Og -g3' PKG_CONFIG_PATH=/opt/qt5/lib/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD PDUMPER LCMS2

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix