refactor: simplify code in AddMissingPort

[lgbgbl]

What type of PR is this?

refactor

What this PR does / why we need it (English/Chinese):

en:

1. When the length of substr is 1, we can call strings.IndexByte directly rather than calling len() for substr and then calling strings.IndexByte in strings.Index.
2. We can assign variable as string type instead of int type so that we are able to leave out the work about converting integer to string.
3. net.JoinHostPort() will judge whether : is in addr again, and will concat host and port if : is not in addr. However, when we call net.JoinHostPort in AddingMissingPort, : should not in addr. So we can also concat host and port directly instead of calling net.JoinHostPort.

zh:

1. 当子串长度确定为1时，可以直接调用 strings.IndexByte 函数而不是像 strings.Index 一样先调用 len() 判断子串长度后再调用 strings.IndexByte 函数.
2. 为省去整型数字转字符串的工作，可以将相关变量直接定义成string类型而不是int类型。
3. net包下的 JoinHostPort 函数会再次判断 : 是否在 addr 中，如果不在则将 host 与 port 相关字符串连接起来。然而在 AddingMissingPort 函数中调用 net.JoinHostPort 时，:应不在 addr 中。所以在此可以不调用 net.JoinHostPort，而是直接连接 host 和 port 信息。

Which issue(s) this PR fixes:

None

[welkeyever]

lgtm

[welkeyever]

lgtm