html: add a Fuzz function

Adds a sample Fuzz test function to package html based on
https://github.com/dvyukov/go-fuzz-corpus/blob/master/stdhtml/main.go

Updates #19109
Updates #31309

Change-Id: I8c49fff8f70fc8a8813daf1abf0044752003adbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/174301Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/html/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 html
+
+import (
+	"fmt"
+)
+
+func Fuzz(data []byte) int {
+	v := string(data)
+
+	e := EscapeString(v)
+	u := UnescapeString(e)
+	if v != u {
+		fmt.Printf("v = %q\n", v)
+		fmt.Printf("e = %q\n", e)
+		fmt.Printf("u = %q\n", u)
+		panic("not equal")
+	}
+
+	// As per the documentation, this isn't always equal to v, so it makes
+	// no sense to check for equality. It can still be interesting to find
+	// panics in it though.
+	EscapeString(UnescapeString(v))
+
+	return 0
+}