message: Guard against calling become() when terminated
diff: @@ -762,6 +762,10 @@ auto scheduled_actor::reactivate(mailbox_element& x) -> activation_result {
  // -- behavior management ----------------------------------------------------
  void scheduled_actor::do_become(behavior bhvr, bool discard_old) {
  + if (getf(is_terminated_flag)) {
  + CAF_LOG_WARNING("called become() on a terminated actor");
  + return;
  + }
  if (discard_old && !bhvr_stack_.empty())
  bhvr_stack_.pop_back();
  // request_timeout simply resets the timeout when it's invalid
  