[bug]CMessageExt.GetMessageQueueOffset abnormal

[githublaohu]

BUG REPORT

1. Please describe the issue you observed:
   Get the right value。

• What did you do (The steps to reproduce)?
  MQMessageExt *mqMessageExt = new MQMessageExt(); CMessageExt *messageExt = (CMessageExt*)mqMessageExt; mqMessageExt->setStoreTimestamp(123123); GetMessageStoreTimestamp(messageExt);

• What did you expect to see?
  CMessageExt.GetMessageStoreTimestamp == MQMessageExt.getStoreTimestamp

• What did you see instead?

[ShannonDing]

Fixed.