mark_button: fix missing id

plugins/mark_button/init.php

@@ -18,6 +18,7 @@ class Mark_Button extends Plugin {
 
 	function hook_article_button($line) {
 		$marked_pic = "";
+		$id = $line["id"];
 
 		if (get_pref($this->link, "COMBINED_DISPLAY_MODE")) {
 			if (sql_bool_to_bool($line["marked"])) {