fix:bugfix of err check

[zjbpaul1317]

What type of PR is this?

fix bug

Check the PR title.

• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

(Optional) Translate the PR title into Chinese.

修复bug

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
zh(optional):
当连接不可用的时候，iosend()返回错误；但原本代码依然调用operator.OutputAck(n)，但是如果连接已经关闭了，就会panic。应该先检查错误，再调用

(Optional) Which issue(s) this PR fixes:

#355

(optional) The PR that updates user documentation:

[xiaost]

麻烦先给出了一下具体的错误和复现代码？

[zjbpaul1317]

麻烦先给出了一下具体的错误和复现代码？

#355
var n, err = iosend(operator.FD, bs, barriers[i].ivs, false && supportZeroCopy)
operator.OutputAck(n)
if err != nil {
p.appendHup(operator)
continue
}
这里应该先检查err再执行operator.OutputAck(n)

[xiaost]

@zjbpaul1317 之前的issue当前并没有完整的错误信息，我们无法判断引起的具体原因。
如果你能够复现，麻烦使用我们最新的版本、给出具体的复现代码。谢谢。