nacl: fix zero-length writes

NaCl rejects zero-length write using nil pointer.

R=r
CC=golang-dev
https://golang.org/cl/2237042

src/pkg/syscall/syscall_nacl.go

@@ -8,6 +8,8 @@ package syscall
 
 const OS = "nacl"
 
+var _zero [1]byte // pointer used for zero-length writes
+
 // Auto-generated
 
 //sys	Chmod(path string, mode uint32) (errno int)

src/pkg/syscall/zsyscall_nacl_386.go

-// mksyscall.sh -l32 syscall_nacl.go syscall_nacl_386.go
+// mksyscall.sh -l32 -nacl syscall_nacl.go syscall_nacl_386.go
 // MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT
 
 package syscall
@@ -35,11 +35,13 @@ func Fstat(fd int, stat *Stat_t) (errno int) {
 }
 
 func Getdents(fd int, buf []byte) (n int, errno int) {
-	var _p0 *byte
+	var _p0 unsafe.Pointer
 	if len(buf) > 0 {
-		_p0 = &buf[0]
+		_p0 = unsafe.Pointer(&buf[0])
+	} else {
+		_p0 = unsafe.Pointer(&_zero[0])
 	}
-	r0, _, e1 := Syscall(SYS_GETDENTS, uintptr(fd), uintptr(unsafe.Pointer(_p0)), uintptr(len(buf)))
+	r0, _, e1 := Syscall(SYS_GETDENTS, uintptr(fd), uintptr(_p0), uintptr(len(buf)))
 	n = int(r0)
 	errno = int(e1)
 	return
@@ -65,11 +67,13 @@ func Open(path string, mode int, perm uint32) (fd int, errno int) {
 }
 
 func Read(fd int, p []byte) (n int, errno int) {
-	var _p0 *byte
+	var _p0 unsafe.Pointer
 	if len(p) > 0 {
-		_p0 = &p[0]
+		_p0 = unsafe.Pointer(&p[0])
+	} else {
+		_p0 = unsafe.Pointer(&_zero[0])
 	}
-	r0, _, e1 := Syscall(SYS_READ, uintptr(fd), uintptr(unsafe.Pointer(_p0)), uintptr(len(p)))
+	r0, _, e1 := Syscall(SYS_READ, uintptr(fd), uintptr(_p0), uintptr(len(p)))
 	n = int(r0)
 	errno = int(e1)
 	return
@@ -89,11 +93,13 @@ func Stat(path string, stat *Stat_t) (errno int) {
 }
 
 func Write(fd int, p []byte) (n int, errno int) {
-	var _p0 *byte
+	var _p0 unsafe.Pointer
 	if len(p) > 0 {
-		_p0 = &p[0]
+		_p0 = unsafe.Pointer(&p[0])
+	} else {
+		_p0 = unsafe.Pointer(&_zero[0])
 	}
-	r0, _, e1 := Syscall(SYS_WRITE, uintptr(fd), uintptr(unsafe.Pointer(_p0)), uintptr(len(p)))
+	r0, _, e1 := Syscall(SYS_WRITE, uintptr(fd), uintptr(_p0), uintptr(len(p)))
 	n = int(r0)
 	errno = int(e1)
 	return