windows: allow calling WSASendto with nil Sockaddr

Same as CL 434535 did for the syscall package.

For golang/go#55845

Change-Id: I17f30152ae973b64ac65e08cefd5442e9bf19e2c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/435777
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

windows/syscall_windows.go

@@ -1115,9 +1115,13 @@ func Shutdown(fd Handle, how int) (err error) {
 }
 
 func WSASendto(s Handle, bufs *WSABuf, bufcnt uint32, sent *uint32, flags uint32, to Sockaddr, overlapped *Overlapped, croutine *byte) (err error) {
-	rsa, l, err := to.sockaddr()
-	if err != nil {
-		return err
+	var rsa unsafe.Pointer
+	var l int32
+	if to != nil {
+		rsa, l, err = to.sockaddr()
+		if err != nil {
+			return err
+		}
 	}
 	return WSASendTo(s, bufs, bufcnt, sent, flags, (*RawSockaddrAny)(unsafe.Pointer(rsa)), l, overlapped, croutine)
 }