debuggerd: fix `debuggerd <pid>` am: 1e4afb52

  std::thread redirect_thread = spawn_redirect_thread(std::move(piperead));

  bool backtrace = argc == 3;

  if (!debuggerd_trigger_dump(pid, std::move(pipewrite),

                              backtrace ? kDebuggerdBacktrace : kDebuggerdBacktrace, 0)) {

                              backtrace ? kDebuggerdBacktrace : kDebuggerdTombstone, 0)) {

    redirect_thread.join();

    errx(1, "failed to dump process %d", pid);

  }