Upstream patch to fix "md5sum --check --ignore-missing" which treated
files with checksums starting with "00" as missing.
To be removed with coreutils > v8.25.

Test case illustrating the issue:

  $ echo 559 > file
  
  $ md5sum file | tee file.md5
  000b64c5d808b7ae98718d6a191325b7  file
  
With the file being intact, md5sum previously issued a misleading error
diagnostic, and returned with exit code 1.

  $ md5sum -c --ignore-missing file.md5; echo $?
  md5sum: file.md5: no file was verified
  1

With the file being changed, md5sum still detected the changed file ("FAILED"),
but still stated that the file was not checked; at least the exit code was okay.

  $ echo tampered >> file

  $ md5sum -c --ignore-missing file.md5; echo $?
  file: FAILED
  md5sum: WARNING: 1 computed checksum did NOT match
  md5sum: file.md5: no file was verified
  1

Original patch:
http://git.sv.gnu.org/cgit/coreutils.git/commit/?id=d0ddfadfb27de

From d0ddfadfb27def2861f35b1a45190a4c1780b257 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A1draig=20Brady?= <P@draigBrady.com>
Date: Wed, 26 Oct 2016 15:45:01 +0100
Subject: [PATCH] md5sum,sha*sum: fix --ignore-missing with checksums starting
 with 00

* NEWS: Mention the fix.
* src/md5sum.c (digest_file): Add a new MISSING parameter to
return whether the file was missing, separately from the digest.
* tests/misc/md5sum.pl: Add a test case.
Fixes http://bugs.gnu.org/24795
---
 NEWS                 |    4 ++++
 src/md5sum.c         |   20 ++++++++++++--------
 tests/misc/md5sum.pl |    7 +++++++
 3 files changed, 23 insertions(+), 8 deletions(-)

Index: NEWS
===================================================================
--- NEWS.orig
+++ NEWS
@@ -11,6 +11,10 @@ GNU coreutils NEWS
   df now filters the system mount list more efficiently, with 20000
   mount entries now being processed in about 1.1s compared to 1.7s.
 
+  md5sum --check --ignore-missing no longer treats files with checksums
+  starting with "00" as missing.  This also affects sha*sum.
+  [bug introduced with the --ignore-missing feature in coreutils-8.25]
+
 
 * Noteworthy changes in release 8.25 (2016-01-20) [stable]
 
Index: src/md5sum.c
===================================================================
--- src/md5sum.c.orig
+++ src/md5sum.c
@@ -462,15 +462,19 @@ print_filename (char const *file, bool e
    text because it was a terminal.
 
    Put the checksum in *BIN_RESULT, which must be properly aligned.
+   Put true in *MISSING if the file can't be opened due to ENOENT.
    Return true if successful.  */
 
 static bool
-digest_file (const char *filename, int *binary, unsigned char *bin_result)
+digest_file (const char *filename, int *binary, unsigned char *bin_result,
+             bool *missing)
 {
   FILE *fp;
   int err;
   bool is_stdin = STREQ (filename, "-");
 
+  *missing = false;
+
   if (is_stdin)
     {
       have_read_stdin = true;
@@ -490,7 +494,7 @@ digest_file (const char *filename, int *
         {
           if (ignore_missing && errno == ENOENT)
             {
-              *bin_result = '\0';
+              *missing = true;
               return true;
             }
           error (0, errno, "%s", quotef (filename));
@@ -603,14 +607,14 @@ digest_check (const char *checkfile_name
                                           '8', '9', 'a', 'b',
                                           'c', 'd', 'e', 'f' };
           bool ok;
+          bool missing;
           /* Only escape in the edge case producing multiple lines,
              to ease automatic processing of status output.  */
           bool needs_escape = ! status_only && strchr (filename, '\n');
 
           properly_formatted_lines = true;
 
-          *bin_buffer = '\1'; /* flag set to 0 for ignored missing files.  */
-          ok = digest_file (filename, &binary, bin_buffer);
+          ok = digest_file (filename, &binary, bin_buffer, &missing);
 
           if (!ok)
             {
@@ -623,10 +627,9 @@ digest_check (const char *checkfile_name
                   printf (": %s\n", _("FAILED open or read"));
                 }
             }
-          else if (ignore_missing && ! *bin_buffer)
+          else if (ignore_missing && missing)
             {
-              /* Treat an empty buffer as meaning a missing file,
-                 which is ignored with --ignore-missing.  */
+              /* Ignore missing files with --ignore-missing.  */
               ;
             }
           else
@@ -876,8 +879,9 @@ main (int argc, char **argv)
       else
         {
           int file_is_binary = binary;
+          bool missing;
 
-          if (! digest_file (file, &file_is_binary, bin_buffer))
+          if (! digest_file (file, &file_is_binary, bin_buffer, &missing))
             ok = false;
           else
             {
Index: tests/misc/md5sum.pl
===================================================================
--- tests/misc/md5sum.pl.orig
+++ tests/misc/md5sum.pl
@@ -149,6 +149,13 @@ my @Tests =
                                 {ERR=>
                                     "md5sum: f.md5: no file was verified\n"},
                                 {EXIT=> 1}],
+     # coreutils-8.25 with --ignore-missing treated checksums starting with 00
+     # as if the file was not present
+     ['check-ignore-missing-6', '--check', '--ignore-missing',
+                                {AUX=> {f=> '9t'}},
+                                {IN=> {'f.md5' =>
+                                    "006999e6df389641adf1fa3a74801d9d  f\n"}},
+                                {OUT=>"f: OK\n"}],
      ['bsd-segv', '--check', {IN=> {'z' => "MD5 ("}}, {EXIT=> 1},
       {ERR=> "$prog: z: no properly formatted MD5 checksum lines found\n"}],