Added test case for broken merge on empty undo_db

tests/tests/database_tests.cpp

@@ -59,3 +59,22 @@ BOOST_AUTO_TEST_CASE( undo_test )
       throw;
    }
 }
+
+BOOST_AUTO_TEST_CASE( merge_test )
+{
+   try {
+      database db;
+      auto ses = db._undo_db.start_undo_session();
+      const auto& bal_obj1 = db.create<account_balance_object>( [&]( account_balance_object& obj ){
+          obj.balance = 42;
+      });
+      ses.merge();
+
+      auto balance = db.get_balance( account_id_type(), asset_id_type() );
+      BOOST_CHECK_EQUAL( 42, balance.amount.value );
+   } catch ( const fc::exception& e )
+   {
+      edump( (e.to_detail_string()) );
+      throw;
+   }
+}