message: ChainLength::next(): Use checked_add just to be safe
diff: @@ -179,7 +179,7 @@ impl Header {
  impl property::ChainLength for ChainLength {
  fn next(&self) -> Self {
  - ChainLength(self.0 + 1)
  + ChainLength(self.0.checked_add(1).unwrap())
  }
  }
  