message: Fix possible overflow
diff: @@ -133,7 +133,7 @@ public:
  create_thread();
  }
  //--
  - else if (n_waiting - n_closing > min_threads)
  + else if (n_waiting > n_closing + min_threads)
  {
  thread_context *tc = &threads.front();
  *tc->_exit_flag = true;
  