message: core: fix segfault on plugin init
  This fixes the case where on plugin initialization, the std::vector
  _mavlink_handler_table is growing and needs to be re-located, however,
  the iterator is not invalidated as it should because everything moved
  and we're accessing garbage.
diff: @@ -76,6 +76,8 @@ void SystemImpl::register_mavlink_message_handler(uint16_t msg_id,
  MAVLinkHandlerTableEntry entry = {msg_id, callback, cookie};
  _mavlink_handler_table.push_back(entry);
  + // Push back can move the std::vector and therefore invalidate the iterator.
  + _iterator_invalidated = true;
  }
  void SystemImpl::unregister_all_mavlink_message_handlers(const void *cookie)
  