message: Fix semantics of property getter
diff: @@ -75,10 +75,11 @@ public:
  return central_buf + max_path_buf;
  }
  + /// Returns the number of buffered elements for this specific slot, ignoring
  + /// the central buffer.
  size_t buffered(stream_slot slot) const noexcept override {
  auto i = state_map_.find(slot);
  - return this->buf_.size()
  - + (i != state_map_.end() ? i->second.buf.size() : 0u);
  + return i != state_map_.end() ? i->second.buf.size() : 0u;
  }
  int32_t max_capacity() const noexcept override {
  