message: [dbus] fix using `dbusError` after free
  dbusError is freed before used in logging. This caused wrong logging message.
diff: @@ -64,11 +64,11 @@ otbrError DBusAgent::Init(void)
  mThreadObject = std::unique_ptr<DBusThreadObject>(new DBusThreadObject(mConnection.get(), mInterfaceName, mNcp));
  error = mThreadObject->Init();
  exit:
  - dbus_error_free(&dbusError);
  if (error != OTBR_ERROR_NONE)
  {
  otbrLog(OTBR_LOG_ERR, "dbus error %s: %s", dbusError.name, dbusError.message);
  }
  + dbus_error_free(&dbusError);
  return error;
  }
  