net/rpc/jsonrpc: nil pointer deference on invalid reply.

Fixes #5006.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7691045

src/pkg/net/rpc/jsonrpc/all_test.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net"
 	"net/rpc"
 	"testing"
@@ -185,6 +186,22 @@ func TestMalformedInput(t *testing.T) {
 	ServeConn(srv)                 // must return, not loop
 }
 
+func TestMalformedOutput(t *testing.T) {
+	cli, srv := net.Pipe()
+	go srv.Write([]byte(`{"id":0,"result":null,"error":null}`))
+	go ioutil.ReadAll(srv)
+
+	client := NewClient(cli)
+	defer client.Close()
+
+	args := &Args{7, 8}
+	reply := new(Reply)
+	err := client.Call("Arith.Add", args, reply)
+	if err == nil {
+		t.Error("expected error")
+	}
+}
+
 func TestUnexpectedError(t *testing.T) {
 	cli, srv := myPipe()
 	go cli.PipeWriter.CloseWithError(errors.New("unexpected error!")) // reader will get this error

src/pkg/net/rpc/jsonrpc/client.go

@@ -83,7 +83,7 @@ func (c *clientCodec) ReadResponseHeader(r *rpc.Response) error {
 
 	r.Error = ""
 	r.Seq = c.resp.Id
-	if c.resp.Error != nil {
+	if c.resp.Error != nil || c.resp.Result == nil {
 		x, ok := c.resp.Error.(string)
 		if !ok {
 			return fmt.Errorf("invalid error %v", c.resp.Error)