[ISSUE #1013] Fix: select one message queue of different broker when retry to send

[cserwen]

What is the purpose of the change

fix #1013

Brief changelog

XX

Verifying this change

XXXX

Follow this checklist to help us incorporate your contribution quickly and easily. Notice, it would be helpful if you could finish the following 5 checklist(the last one is not necessary)before request the community to review your PR.

• Make sure there is a Github issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a Github issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [ISSUE #123] Fix UnknownException when host config not exist. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test(over 80% coverage) to verify your logic correction, more mock a little better when a cross-module dependency exists.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

[francisoliverlee]

建议添加一些描述， 说明bug出现的场景和修复方法。方便大家review和理解代码逻辑

[cserwen]

建议添加一些描述， 说明bug出现的场景和修复方法。方便大家review和理解代码逻辑

@francisoliverlee Hi, please see issue #1013.

[twz915]

@cserwen

mq := p.selectMessageQueue(msg, lastBrokerName)

this is a bug, which should be

mq = p.selectMessageQueue(msg, lastBrokerName)

fixed https://github.com/apache/rocketmq-client-go/pull/1111/files#diff-66ce6ea3bd431c5246882c1e7f5e03cadf98171017e9f338bc5acdb703a3d579L332