message: Fix integer sign warning
diff: @@ -50,7 +50,7 @@ error stream_source::downstream_demand(strong_actor_ptr& hdl, long value) {
  auto current_size = buf_size();
  auto size_hint = out().total_net_credit();
  if (current_size < size_hint)
  - generate(size_hint - current_size);
  + generate(static_cast<size_t>(size_hint - current_size));
  return push(&size_hint);
  }
  // transmit cached elements before closing paths
  @@ -71,7 +71,7 @@ void stream_source::generate() {
  auto current_size = buf_size();
  auto size_hint = out().total_net_credit();
  if (current_size < size_hint)
  - generate(size_hint - current_size);
  + generate(static_cast<size_t>(size_hint - current_size));
  }
  }
  