fix: prevent signed/unsigned mix in TcpConn::isend

[VecherVhatuX]

In TcpConn::isend, the variable sended is size_t (unsigned), while wd returned from writeImp is ssize_t (signed). The original code sended += wd; mixes signed and unsigned types. If wd is negative (e.g., -1 on error), adding it to sended would convert it to a large positive value, potentially causing wraparound or logic errors.

This is a classic case of CWE-190: Integer Overflow or Wraparound
(https://cwe.mitre.org/data/definitions/190.html).

Fix:

• Only add wd to sended when wd > 0
• Ensures that no signed value is implicitly converted to unsigned
• Keeps original logic intact for normal write operations

Importance:

• Prevents accidental large values for sended
• Ensures correct accounting of bytes sent
• Avoids potential memory or buffer misuse

[ggandycong]

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[VecherVhatuX]

@ggandycong, could you please tell whether this PR is ok or should we just close the PR?

[ggandycong]

刘德聪，已经收到你的邮件，请你放心 祝你生活愉快，笑口常开