message: fix compiler error on gcc 5.x
diff: @@ -91,7 +91,7 @@ struct auth_provider
  [](const std::string&){ return "no_auth_required"; },
  [](const std::string&, const std::string&) {
  return auth_plan{mode::open,{}}; },
  - nullptr, nullptr };
  + nullptr, nullptr, nullptr };
  }
  };
  