encoding/json: Fix missing error when trying to unmarshal null string into int, for successive ,string option

Fixes #7046.

R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/47260043

src/pkg/encoding/json/decode_test.go

@@ -1060,6 +1060,21 @@ func TestEmptyString(t *testing.T) {
 	}
 }
 
+// Test that the returned error is non-nil when trying to unmarshal null string into int, for successive ,string option
+// Issue 7046
+func TestNullString(t *testing.T) {
+	type T struct {
+		A int `json:",string"`
+		B int `json:",string"`
+	}
+	data := []byte(`{"A": "1", "B": null}`)
+	var s T
+	err := Unmarshal(data, &s)
+	if err == nil {
+		t.Fatalf("expected error; got %v", s)
+	}
+}
+
 func intp(x int) *int {
 	p := new(int)
 	*p = x
@@ -1110,8 +1125,8 @@ func TestInterfaceSet(t *testing.T) {
 // Issue 2540
 func TestUnmarshalNulls(t *testing.T) {
 	jsonData := []byte(`{
-		"Bool"    : null, 
-		"Int"     : null, 
+		"Bool"    : null,
+		"Int"     : null,
 		"Int8"    : null,
 		"Int16"   : null,
 		"Int32"   : null,