Improve fuzzing function

We not only want to prevent panics, we also want rendering org -> org to not
change the meaning of the file. One easy way to check that (for the
nodes that print to html) whether that holds is to compare the html output.

org/fuzz.go

@@ -2,18 +2,25 @@
 
 package org
 
-import "bytes"
+import (
+	"bytes"
+	"strings"
+)
 
 // Fuzz function to be used by https://github.com/dvyukov/go-fuzz
-func Fuzz(data []byte) int {
-	d := NewDocument().Silent().Parse(bytes.NewReader(data))
-	_, err := d.Write(NewOrgWriter())
+func Fuzz(input []byte) int {
+	d := NewDocument().Silent().Parse(bytes.NewReader(input))
+	orgOutput, err := d.Write(NewOrgWriter())
 	if err != nil {
 		panic(err)
 	}
-	_, err = d.Write(NewHTMLWriter())
+	htmlOutputA, err := d.Write(NewHTMLWriter())
 	if err != nil {
 		panic(err)
 	}
+	htmlOutputB, err := NewDocument().Silent().Parse(strings.NewReader(orgOutput)).Write(NewHTMLWriter())
+	if htmlOutputA != htmlOutputB {
+		panic("rendered org results in different html than original input")
+	}
 	return 0
 }