message: Add another debug assert in mark_as_acked
  Just to make sure.
diff: @@ -340,6 +340,7 @@ impl TxBuffer {
  // We can drop contig acked range from the buffer
  let new_retirable = self.ranges.acked_from_zero() - self.retired;
  + debug_assert!(new_retirable <= self.buffered() as u64);
  let keep_len =
  self.buffered() - usize::try_from(new_retirable).expect("should fit in usize");
  