fix only_v6() panic on windows

[zh-jq]

I have encountered a panic case in only_v6 in github CI on windows:
https://github.com/bytedance/g3/actions/runs/16095794585/job/45418469197

The input buffer size (optlen) must be 4, and the output optlen may be 1. As the input optval is initialized to zero, it will be modified to nonzero when ipv6 only, so just use optval != 0 to get the result value.

[Thomasdezeeuw]

We tried this already, see #593 and #564, it fails on our Windows CI.

[zh-jq]

We tried this already, see #593 and #564, it fails on our Windows CI.

It seems that the input buffer size must be 4 bytes, and the output optlen may be 1, so the check could be: it will be ipv6 only if any of the buffer bits are not zero.

[ChrisDenton]

To be clear, getsockopt is enforcing that the input buffer is at least DWORD in size (aka 4 bytes) but is only writing a single BOOLEAN (1 byte) into the buffer. Therefore the assertion ends up being too strong for this particular case. This is somewhat surprising but is technically allowed by the API.