syscall: check if to is nil to prevent panic from WSASendto

syscall: check if to is nil to prevent panic from WSASendto

to is an optional pointer to sockaddr, as written in the doc:
https://learn.microsoft.com/en-us/windows/win32/api/winsock2/nf-winsock2-wsasendto

For #55845

Change-Id: Ia685cec8d9bc9ff313f598db9d2213a1f409757a
Reviewed-on: https://go-review.googlesource.com/c/go/+/434535
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: xie cui <523516579@qq.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>