message: storage-units: Use checked arithmetic in calculating file position
  Prevent any possibility of integer overflow when tracking file position.
diff: @@ -112,7 +112,9 @@ impl<R: Read> Reader<R> {
  None => {}
  Some(ref data) => {
  self.hash_context.input(data);
  - self.pos += 4 + offset_align4(data.len() as u64);
  + self.pos = self.pos
  + .checked_add(4).unwrap()
  + .checked_add(offset_align4(data.len() as u64)).unwrap();
  }
  };
  mdata
  @@ -182,7 +184,10 @@ impl Writer {
  0
  };
  self.index.append(blockhash, self.pos);
  - self.pos += 4 + len as u64 + pad_bytes as u64;
  + self.pos = self.pos
  + .checked_add(4).unwrap()
  + .checked_add(len as u64).unwrap()
  + .checked_add(pad_bytes as u64).unwrap();
  self.nb_blobs += 1;
  Ok(())
  }
  