encoding/json: address go vet reports

The error message for decoding a unquoted value into a struct field with
the ,string option specified has two arguments when one is needed.
Make the error message take one argument and add a test in order to cover
the case when a unquoted value is specified.

Also add error value as the missing argument for Fatalf call in test.

Fixes the following go vet reports:

decode.go:602: wrong number of args for format in Errorf call: 1 needed but 2 args
decode_test.go:1088: missing argument for Fatalf("%v"): format reads arg 1, have only 0 args

Change-Id: Id036e10c54c4a7c1ee9952f6910858ecc2b84134
Reviewed-on: https://go-review.googlesource.com/2109Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/encoding/json/decode.go

@@ -599,7 +599,7 @@ func (d *decodeState) object(v reflect.Value) {
 			case string:
 				d.literalStore([]byte(qv), subv, true)
 			default:
-				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal unquoted value into %v", item, v.Type()))
+				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal unquoted value into %v", subv.Type()))
 			}
 		} else {
 			d.value(subv)

src/encoding/json/decode_test.go

@@ -688,6 +688,7 @@ var wrongStringTests = []wrongStringTest{
 	{`{"result":"x"}`, `json: invalid use of ,string struct tag, trying to unmarshal "x" into string`},
 	{`{"result":"foo"}`, `json: invalid use of ,string struct tag, trying to unmarshal "foo" into string`},
 	{`{"result":"123"}`, `json: invalid use of ,string struct tag, trying to unmarshal "123" into string`},
+	{`{"result":123}`, `json: invalid use of ,string struct tag, trying to unmarshal unquoted value into string`},
 }
 
 // If people misuse the ,string modifier, the error message should be
@@ -1085,7 +1086,7 @@ func TestNullString(t *testing.T) {
 	*s.C = 2
 	err := Unmarshal(data, &s)
 	if err != nil {
-		t.Fatalf("Unmarshal: %v")
+		t.Fatalf("Unmarshal: %v", err)
 	}
 	if s.B != 1 || s.C != nil {
 		t.Fatalf("after Unmarshal, s.B=%d, s.C=%p, want 1, nil", s.B, s.C)