fs/iso9660: Set a grub_errno if mount fails

It was possible for a grub_errno to not be set if mount of an ISO 9660
filesystem failed when set_rockridge() returned 0.

This isn't known to be exploitable as the other filesystems due to
filesystem helper checking the requested file type. Though fixing
as a precaution.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/iso9660.c

@@ -551,6 +551,9 @@ grub_iso9660_mount (grub_disk_t disk)
   return data;
 
  fail:
+  if (grub_errno == GRUB_ERR_NONE)
+    grub_error (GRUB_ERR_BAD_FS, "not a ISO9660 filesystem");
+
   grub_free (data);
   return 0;
 }