inet_ntop: avoid the strlen()

[bagder]

Also, skip adding the terminating null that is not used.

[Copilot]

Pull request overview

This PR optimizes the inet_ntop6 function by eliminating an unnecessary strlen() call and removing the redundant null terminator that was being added to the temporary buffer. The optimization leverages pointer arithmetic that is already available from the string construction process.

Key changes:

• Removed the null terminator assignment to the temporary buffer since curlx_strcopy handles null termination
• Updated overflow check from > to >= to correctly validate buffer space requirements
• Replaced strlen(tmp) with tp - tmp to use the already-computed string length

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