message: Fix null pointer dereference
  Although it has fewer chance chain_plugin is invalid, still it's safer to assert it.
diff: @@ -51,7 +51,7 @@ public:
  void applied_block(const chain::block_trace&);
  fc::variant transaction_to_variant(const packed_transaction& pretty_input) const;
  - chain_plugin* chain_plug;
  + chain_plugin* chain_plug = nullptr;
  static const int64_t DEFAULT_TRANSACTION_TIME_LIMIT;
  int64_t transactions_time_limit = DEFAULT_TRANSACTION_TIME_LIMIT;
  std::set<account_name> filter_on;
  @@ -479,6 +479,7 @@ void account_history_plugin::plugin_initialize(const variables_map& options)
  }
  my->chain_plug = app().find_plugin<chain_plugin>();
  + EOS_ASSERT( my->chain_plug, chain::missing_chain_plugin_exception, "" );
  my->chain_plug->chain_config().applied_block_callbacks.emplace_back(
  [&impl = my](const chain::block_trace& trace) {
  impl->check_init_db();
  