html: Fix html writer footnotes (in headlines)

writer.footnotes must be a pointer as we copy the writer in nodesAsString() and
can thus end up modifying the footnotes.list slice without it being reflected in
the original writer (i.e. when the backing array of the slice changes).

org/html_writer.go

@@ -19,7 +19,7 @@ type HTMLWriter struct {
 	HighlightCodeBlock func(source, lang string) string
 	htmlEscape         bool
 	log                *log.Logger
-	footnotes          footnotes
+	footnotes          *footnotes
 }
 
 type footnotes struct {
@@ -61,7 +61,7 @@ func NewHTMLWriter() *HTMLWriter {
 		HighlightCodeBlock: func(source, lang string) string {
 			return fmt.Sprintf("<div class=\"highlight\">\n<pre>\n%s\n</pre>\n</div>", html.EscapeString(source))
 		},
-		footnotes: footnotes{
+		footnotes: &footnotes{
 			mapping: map[string]int{},
 		},
 	}