> +    /* Round up to the nearest sector boundary, and zero the extra memory */
>      core_img = realloc (core_img, nsec * GRUB_DISK_SECTOR_SIZE);
> +    assert (core_img && (nsec * GRUB_DISK_SECTOR_SIZE >= core_size));
Can you change that to xrealloc while on it?
Other than this, go ahead.