/* nthibr.c - tests whether an MS Windows system partition is hibernated */
/*
  *  GRUB  --  GRand Unified Bootloader
  *  Copyright (C) 2007,2008,2009,2011  Free Software Foundation, Inc.
  *
  *  GRUB is free software: you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
  *  the Free Software Foundation, either version 3 of the License, or
  *  (at your option) any later version.
  *
  *  GRUB is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License
  *  along with GRUB.  If not, see<http://www.gnu.org/licenses/>.
  */

#include<grub/types.h>
#include<grub/mm.h>
#include<grub/disk.h>
#include<grub/file.h>
#include<grub/misc.h>
#include<grub/dl.h>
#include<grub/extcmd.h>
#include<grub/lib/arg.h>
#include<grub/err.h>
#include<grub/i18n.h>

GRUB_MOD_LICENSE("GPLv3+");

/* Define this 'empty' array to let the '-h' and '-u' switches be processed */
static const struct grub_arg_option options[] = {
   {0, 0, 0, 0, 0, 0}
};

Please remove this and use grub_command_t

static grub_err_t
grub_cmd_nthibr (grub_extcmd_context_t ctxt __attribute__ ((unused)),
                  int argc, char **args)
{
   char *partition_name = 0, *hibr_file_path = 0, hibr_file_magic[5];
   grub_size_t name_length;
   grub_ssize_t magic_size;
   grub_disk_t partition;
   grub_file_t hibr_file = 0;

   /* Check argument count */
   if (!argc)
     return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("too few arguments"));
   else if (argc>  1)
     return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("too many arguments"));

Either ignore trailing argument or put both in one check != 1

   /* Make copy of partition specifier, so it can be modified (if needed) */
   name_length = grub_strlen (args[0]);
   partition_name = grub_zalloc (name_length + 3);
   if (!partition_name)
     goto exit;
   grub_strcpy (partition_name, args[0]);

   /* Ensure partition specifier start with a '(' */
   if (partition_name[0] != '(')
     {
       grub_memmove (partition_name + 1, partition_name, name_length++);
       partition_name[0] = '(';
     }

   /* Ensure partition specifier ends with a ')' */
   if (partition_name[name_length - 1] != ')')
     partition_name[(++name_length) - 1] = ')';

   /* Check if partition actually exists */
   partition_name[name_length - 1] = '\0';
   partition = grub_disk_open (partition_name + 1);
   if (!partition)
     goto exit;
   grub_disk_close (partition);
   partition_name[name_length - 1] = ')';

   /* Build path to 'hiberfil.sys' */
   hibr_file_path = grub_malloc ((grub_strlen (partition_name)
                                  + grub_strlen ("/hiberfil.sys") + 1));
   if (!hibr_file_path)
     goto exit;
   grub_strcpy (hibr_file_path, partition_name);
   grub_strcat (hibr_file_path, "/hiberfil.sys");

It would be more efficient if you just try to open file and see what error you get. Actually you don't even need to distinguish between error cases here since you return the error you encountered. This will also save one useless alloc.

   /* Try to open 'hiberfil.sys' */
   hibr_file = grub_file_open (hibr_file_path);
   if (!hibr_file)
     {
       if (grub_errno == GRUB_ERR_FILE_NOT_FOUND)
         grub_error (GRUB_ERR_FILE_NOT_FOUND, N_("'hiberfil.sys' not found"));

This overriding is unnecessary. FS code already provides this message

       goto exit;
     }

   /* Try to read magic number of 'hiberfil.sys' */
   magic_size = sizeof (hibr_file_magic) - 1;
   grub_memset (hibr_file_magic, 0, sizeof (hibr_file_magic));
   if (grub_file_read (hibr_file, hibr_file_magic, magic_size)<  magic_size)
     {
       if (!grub_errno)
         grub_error (GRUB_ERR_BAD_FILE_TYPE, N_("'hiberfil.sys' too small"));
       goto exit;
     }

   /* Return SUCCESS if magic indicates file is active; else return FAILURE */
   if (!grub_strncasecmp ("hibr", hibr_file_magic, magic_size))
     grub_error (GRUB_ERR_NONE, "true");
   else
     grub_error (GRUB_ERR_TEST_FAILURE, "false");

exit:
   /* Ensure unmanaged resources are cleaned up */
   if (partition_name)
     grub_free (partition_name);
   if (hibr_file_path)
     grub_free (hibr_file_path);

No need to check that argument to free is non-NULL.

   if (hibr_file)
     grub_file_close (hibr_file);

   return grub_errno;
}

static grub_extcmd_t cmd;

GRUB_MOD_INIT (nthibr)
{
   cmd = grub_register_extcmd ("nthibr", grub_cmd_nthibr, 0,
                               N_("DEVICE"),
                               N_("Test whether an NT system partition "
                                  "is hibernated."),
                               options);
}

GRUB_MOD_FINI (nthibr)
{
   grub_unregister_extcmd (cmd);
}