unix: fix sendmsgN return value for empty iovecs and non-empty oob

Don't shadow the empty var when determining whether to send a single
byte when iovecs are empty but oob is non-empty. This will lead to the
n value correctly being reset to 0 before return.

No test because it's not possible to trigger this case on all platforms,
e.g. darwin where sendmsg with empty buf and non-empty oob returns
EINVAL.

This was introduced by CL 412497 and CL 419396.

Updates golang/go#52885

Change-Id: Iafc5a4b22e10b396ba5f7d4f2ac1c50df195a125
Reviewed-on: https://go-review.googlesource.com/c/sys/+/419914
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

unix/syscall_aix.go

@@ -253,7 +253,7 @@ func sendmsgN(fd int, iov []Iovec, oob []byte, ptr unsafe.Pointer, salen _Sockle
 	var empty bool
 	if len(oob) > 0 {
 		// send at least one normal byte
-		empty := emptyIovecs(iov)
+		empty = emptyIovecs(iov)
 		if empty {
 			var iova [1]Iovec
 			iova[0].Base = &dummy

unix/syscall_bsd.go

@@ -363,7 +363,7 @@ func sendmsgN(fd int, iov []Iovec, oob []byte, ptr unsafe.Pointer, salen _Sockle
 	var empty bool
 	if len(oob) > 0 {
 		// send at least one normal byte
-		empty := emptyIovecs(iov)
+		empty = emptyIovecs(iov)
 		if empty {
 			var iova [1]Iovec
 			iova[0].Base = &dummy

unix/syscall_linux.go

@@ -1541,7 +1541,7 @@ func sendmsgN(fd int, iov []Iovec, oob []byte, ptr unsafe.Pointer, salen _Sockle
 	var dummy byte
 	var empty bool
 	if len(oob) > 0 {
-		empty := emptyIovecs(iov)
+		empty = emptyIovecs(iov)
 		if empty {
 			var sockType int
 			sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)