Fixed undefined property (#346)

library/HTMLPurifier/Printer/ConfigForm.php

@@ -32,6 +32,11 @@ class HTMLPurifier_Printer_ConfigForm extends HTMLPurifier_Printer
      */
     protected $compress = false;
 
+    /**
+     * @var HTMLPurifier_Config
+     */
+    protected $genConfig;
+
     /**
      * @param string $name Form element name for directives to be stuffed into
      * @param string $doc_url String documentation URL, will have fragment tagged on