ZFS: Don't iterate over null objsets

Reading them is harmless but useless as they are empty by definition

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/zfs/zfs.c

@@ -4157,6 +4157,9 @@ iterate_zap_fs (const char *name, grub_uint64_t val,
   grub_err_t err;
   struct grub_dirhook_info info;
 
+  if (name[0] == 0 && val == 0)
+    return 0;
+
   dnode_end_t mdn;
   err = dnode_get (&(ctx->data->mos), val, 0, &mdn, ctx->data);
   if (err)