efi: properly terminate filepath with NULL in chainloader

EFI File Path Media Device Path is defined as NULL terminated string;
but chainloader built file paths without final NULL. This caused error
with Secure Boot and Linux Foundation PreLoader on Acer with InsydeH20 BIOS.
Apparently firmware failed verification with EFI_INVALID_PARAMETER which is
considered fatal error by PreLoader.

Reported and tested by Giovanni Santini <itachi.sama.amaterasu@gmail.com>

grub-core/loader/efi/chainloader.c

@@ -122,6 +122,8 @@ copy_file_path (grub_efi_file_path_device_path_t *fp,
     if (*p == '/')
       *p = '\\';
 
+  /* File Path is NULL terminated */
+  fp->path_name[size++] = '\0';
   fp->header.length = size * sizeof (grub_efi_char16_t) + sizeof (*fp);
 }
 
@@ -156,8 +158,10 @@ make_file_path (grub_efi_device_path_t *dp, const char *filename)
       d = GRUB_EFI_NEXT_DEVICE_PATH (d);
     }
 
+  /* File Path is NULL terminated. Allocate space for 2 extra characters */
+  /* FIXME why we split path in two components? */
   file_path = grub_malloc (size
-			   + ((grub_strlen (dir_start) + 1)
+			   + ((grub_strlen (dir_start) + 2)
 			      * GRUB_MAX_UTF16_PER_UTF8
 			      * sizeof (grub_efi_char16_t))
 			   + sizeof (grub_efi_file_path_device_path_t) * 2);