Fix error getting permission_object from db

libraries/chain/chain_controller.cpp

@@ -724,7 +724,7 @@ void chain_controller::update_global_properties(const signed_block& b) {
          active_producers_authority.accounts.push_back({{name, config::ActiveName}, 1});
       }
 
-      auto po = _db.get<permission_object, by_owner>( boost::make_tuple(config::ProducersAccountName, config::ActiveName) );
+      auto& po = _db.get<permission_object, by_owner>( boost::make_tuple(config::ProducersAccountName, config::ActiveName) );
       _db.modify(po,[active_producers_authority] (permission_object& po) {
          po.auth = active_producers_authority;
       });