image/png: add Fuzz function

Add a Fuzz function to package png, under the gofuzz build
tag. This function is based on the png/png.go code, from
github.com/dvyukov/go-fuzz-corpus, modified to use direct
comparison of image bounds rather than reflect.DeepEqual.

Updates #30979
Updates #19109

Change-Id: Idb86e7ded0c2d78e6cadbeda84c7b1f35b8c579c
Reviewed-on: https://go-review.googlesource.com/c/go/+/168558Reviewed-by: thepudds <thepudds1460@gmail.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>

src/image/png/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 png
+
+import (
+	"bytes"
+	"fmt"
+)
+
+func Fuzz(data []byte) int {
+	cfg, err := DecodeConfig(bytes.NewReader(data))
+	if err != nil {
+		return 0
+	}
+	if cfg.Width*cfg.Height > 1e6 {
+		return 0
+	}
+	img, err := Decode(bytes.NewReader(data))
+	if err != nil {
+		return 0
+	}
+	levels := []CompressionLevel{
+		DefaultCompression,
+		NoCompression,
+		BestSpeed,
+		BestCompression,
+	}
+	for _, l := range levels {
+		var w bytes.Buffer
+		e := &Encoder{CompressionLevel: l}
+		err = e.Encode(&w, img)
+		if err != nil {
+			panic(err)
+		}
+		img1, err := Decode(&w)
+		if err != nil {
+			panic(err)
+		}
+		got := img1.Bounds()
+		want := img.Bounds()
+		if !got.Eq(want) {
+			fmt.Printf("bounds0: %#v\n", want)
+			fmt.Printf("bounds1: %#v\n", got)
+			panic("bounds have changed")
+		}
+	}
+	return 1
+}