syscall: fix Write(nil) on NaCl

Fixes #7050.

LGTM=crawshaw, r
R=golang-codereviews, crawshaw, r
CC=golang-codereviews
https://golang.org/cl/91590043

src/pkg/syscall/fd_nacl.go

@@ -162,7 +162,13 @@ func Read(fd int, b []byte) (int, error) {
 	return f.impl.read(b)
 }
 
+var zerobuf [0]byte
+
 func Write(fd int, b []byte) (int, error) {
+	if b == nil {
+		// avoid nil in syscalls; nacl doesn't like that.
+		b = zerobuf[:]
+	}
 	f, err := fdToFile(fd)
 	if err != nil {
 		return 0, err

test/fixedbugs/issue7050.go

+// run
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	_, err := os.Stdout.Write(nil)
+	if err != nil {
+		fmt.Printf("BUG: os.Stdout.Write(nil) = %v\n", err)
+	}
+}