unix: add missing address operator in initxattrdest

The purpose of the _zero variable is to provide a valid address for a
pointer to an array of length zero. All other uses of the variable
take its address, but one reference to it (added in CL 147850043)
converts the variable (which has type uintptr) directly to an
unsafe.Pointer, producing a nil pointer instead of a non-nil pointer
to a zero-length array.

This typo is caught by 'go vet', but was masked for a long time by the
numerous false-positive warnings for the same check (#41205).

For golang/go#41205.

Change-Id: Ib3bebfadc6fc5574db19630169ff3f65da857bdd
Reviewed-on: https://go-review.googlesource.com/c/sys/+/465597
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

Author: Bryan C. Mills

unix/xattr_bsd.go

@@ -36,9 +36,14 @@ func xattrnamespace(fullattr string) (ns int, attr string, err error) {
 func initxattrdest(dest []byte, idx int) (d unsafe.Pointer) {
 	if len(dest) > idx {
 		return unsafe.Pointer(&dest[idx])
-	} else {
-		return unsafe.Pointer(_zero)
 	}
+	if dest != nil {
+		// extattr_get_file and extattr_list_file treat NULL differently from
+		// a non-NULL pointer of length zero. Preserve the property of nilness,
+		// even if we can't use dest directly.
+		return unsafe.Pointer(&_zero)
+	}
+	return nil
 }
 
 // FreeBSD and NetBSD implement their own syscalls to handle extended attributes