bug#49723: 28.0.50; Test in coding.c for NUL bytes in filenames is not reliable

[Eli Zaretskii <eliz@gnu.org>]

From: eliz@HOME-C4E4A596F7.i-did-not-set--mail-host-address--so-tickle-me
--text follows this line--
Emacs currently tries to catch null bytes in file names in
encode_file_name, after encoding the file name.  But that is not
reliable enough, because it assumes that the call to expand-file-name
at the beginning of encode_file_name leaves those null bytes intact.
That is not guaranteed, and in fact doesn't happen at least on
MS-Windows: the file name gets chopped after the first null byte, and
doesn't trigger the test in encode_file_name.

So for a more reliable test we need to include such a check inside
expand-file-name (and then we could probably drop the test in
encode_file_name, because Emacs always runs file names through
expand-file-name before using them).

In GNU Emacs 28.0.50 (build 1573, i686-pc-mingw32)
 of 2021-07-24 built on HOME-C4E4A596F7
Repository revision: 7c83e605ab84e8b62254c55f347abc8aa9c6057b
Repository branch: master
Windowing system distributor 'Microsoft Corp.', version 5.1.2600
System Description: Microsoft Windows XP Service Pack 3 (v5.1.0.2600)

Configured using:
 'configure -C --prefix=/d/usr --with-wide-int --with-modules
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -gdwarf-4 -g3''

Configured features:
ACL GIF GMP GNUTLS HARFBUZZ JPEG JSON LCMS2 LIBXML2 MODULES NOTIFY
W32NOTIFY PDUMPER PNG RSVG SOUND THREADS TIFF TOOLKIT_SCROLL_BARS XPM
ZLIB

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1255

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
iso-transl tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win
w32-vars term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads w32notify w32 lcms2 multi-tty
make-network-process emacs)

Memory information:
((conses 16 56888 7103)
 (symbols 48 7785 1)
 (strings 16 21560 2884)
 (string-bytes 1 630636)
 (vectors 16 13320)
 (vector-slots 8 173301 9720)
 (floats 8 23 47)
 (intervals 40 265 116)
 (buffers 888 10))