message: add check to see if object belongs to this multi_index in modify
diff: @@ -668,6 +668,7 @@ class multi_index
  template<typename Lambda>
  void modify( const T& obj, uint64_t payer, Lambda&& updater ) {
  const auto& objitem = static_cast<const item&>(obj);
  + eosio_assert( objitem.__idx == this, "object passed to modify is not in multi_index" );
  auto& mutableitem = const_cast<item&>(objitem);
  auto secondary_keys = boost::hana::transform( _indices, [&]( auto&& idx ) {
  