message: skipp_list get_key bugfix
diff: @@ -159,7 +159,7 @@ impl<K: Codec, V: Codec, C: ListValue<K, V>> TypedSkipList<K, V, C> for Database
  pos = pos.higher();
  lane = lane.higher_lane();
  } else {
  - if pos.base_index() == 0 {
  + if pos.index() == 0 {
  return Ok(None);
  } else {
  pos = pos.prev();
  