unix: allow calling Sendto with nil Sockaddr

Same as CL 433504 did for the syscall package.

For golang/go#55845

Change-Id: I3ca2d7fab9da77c304487b4c134a9675da99deca
Reviewed-on: https://go-review.googlesource.com/c/sys/+/434515
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

Author: tklauser

unix/syscall_unix.go

@@ -423,11 +423,15 @@ func Send(s int, buf []byte, flags int) (err error) {
 }
 
 func Sendto(fd int, p []byte, flags int, to Sockaddr) (err error) {
-	ptr, n, err := to.sockaddr()
-	if err != nil {
-		return err
+	var ptr unsafe.Pointer
+	var salen _Socklen
+	if to != nil {
+		ptr, salen, err = to.sockaddr()
+		if err != nil {
+			return err
+		}
 	}
-	return sendto(fd, p, flags, ptr, n)
+	return sendto(fd, p, flags, ptr, salen)
 }
 
 func SetsockoptByte(fd, level, opt int, value byte) (err error) {