net/rpc/jsonrpc: set exactly one of result or error to non-null in response

Fixes #7442

LGTM=gri
R=golang-codereviews, gri
CC=golang-codereviews
https://golang.org/cl/72570044

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

@@ -205,7 +205,6 @@ func TestMalformedOutput(t *testing.T) {
 }
 
 func TestServerErrorHasNullResult(t *testing.T) {
-	t.Skip("Known failing test; Issue 7442")
 	var out bytes.Buffer
 	sc := NewServerCodec(struct {
 		io.Reader

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

@@ -100,7 +100,6 @@ func (c *serverCodec) ReadRequestBody(x interface{}) error {
 var null = json.RawMessage([]byte("null"))
 
 func (c *serverCodec) WriteResponse(r *rpc.Response, x interface{}) error {
-	var resp serverResponse
 	c.mutex.Lock()
 	b, ok := c.pending[r.Seq]
 	if !ok {
@@ -114,10 +113,9 @@ func (c *serverCodec) WriteResponse(r *rpc.Response, x interface{}) error {
 		// Invalid request so no id.  Use JSON null.
 		b = &null
 	}
-	resp.Id = b
-	resp.Result = x
+	resp := serverResponse{Id: b}
 	if r.Error == "" {
-		resp.Error = nil
+		resp.Result = x
 	} else {
 		resp.Error = r.Error
 	}