encoding/json: add a Fuzz function

Adds a sample Fuzz test function to package encoding/json following the
guidelines defined in #31309, based on
https://github.com/dvyukov/go-fuzz-corpus/blob/master/json/json.go

Fixes #31309
Updates #19109

Change-Id: I5fe04d9a5f41c0de339f8518dae30896ec14e356
Reviewed-on: https://go-review.googlesource.com/c/go/+/174058Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/json/fuzz.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build gofuzz
+
+package json
+
+import (
+	"fmt"
+)
+
+func Fuzz(data []byte) (score int) {
+	for _, ctor := range []func() interface{}{
+		func() interface{} { return new(interface{}) },
+		func() interface{} { return new(map[string]interface{}) },
+		func() interface{} { return new([]interface{}) },
+	} {
+		v := ctor()
+		err := Unmarshal(data, v)
+		if err != nil {
+			continue
+		}
+		score = 1
+
+		m, err := Marshal(v)
+		if err != nil {
+			fmt.Printf("v=%#v\n", v)
+			panic(err)
+		}
+
+		u := ctor()
+		err = Unmarshal(m, u)
+		if err != nil {
+			fmt.Printf("v=%#v\n", v)
+			fmt.Println("m=%s\n", string(m))
+			panic(err)
+		}
+	}
+
+	return
+}