internal/poll: don't pass non-nil WSAMsg.Name with 0 namelen on windows
CL 692436 changed WriteMsgInet{4,6} on windows to pass a zero namelen
when the sockaddr is nil. Turns out Windows also requires name to be
nil when namelen is 0.
With this commit, WriteMsgInet4 and WriteMsgInet6 now nicely align with
WriteMsg.
For #74841
Change-Id: Ifadee2d12d9bce2411f11a0e12b9fa2b3d71990e
Reviewed-on: https://go-review.googlesource.com/c/go/+/698395 Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>