message: Fail poll_once early if no pollset is available
diff: @@ -134,6 +134,8 @@ void multiplexer::close_pipe() {
  }
  bool multiplexer::poll_once(bool blocking) {
  + if (pollset_.empty())
  + return false;
  // We'll call poll() until poll() succeeds or fails.
  for (;;) {
  int presult;
  