Fix logic mixup with newly introduced procedureName variable

When I introduced the new `procedureName` variable to track whether the `Save()` was calling `CreateTopic` or `UpdateTopic` (#ac6c2ba6), I accidentally got the logic wrong. As a result, `CreateTopic` was being called for new topics, and `UpdateTopic` for existing topics—which caused a mismatch with the parameters. Oops. To fix this, I now use the recently introduced `isNew` variable (#36ca32d), which is a bit more semantically intuitive than `topic.Id > 0`.

Author: JeremyCaney

OnTopic.Data.Sql/SqlTopicRepository.cs

@@ -327,13 +327,13 @@ public override int Save([NotNull]Topic topic, bool isRecursive = false, bool is
       /*------------------------------------------------------------------------------------------------------------------------
       | Establish database connection
       \-----------------------------------------------------------------------------------------------------------------------*/
+      var isNew                 = topic.Id == -1;
       var connection            = new SqlConnection(_connectionString);
-      var procedureName         = topic.Id > 0? "CreateTopic" : "UpdateTopic";
+      var procedureName         = isNew? "CreateTopic" : "UpdateTopic";
       var command               = new SqlCommand(procedureName, connection) {
         CommandType             = CommandType.StoredProcedure
       };
       var version               = DateTime.Now;
-      var isNew                 = topic.Id == -1;
 
       /*------------------------------------------------------------------------------------------------------------------------
       | Establish query parameters