ZF-6741: make sure callback is_callable

git-svn-id: http://framework.zend.com/svn/framework/standard/trunk@21970 44c647ce-9c0f-0410-b52a-842ac1e357ba

library/Zend/Form/Decorator/HtmlTag.php

@@ -84,7 +84,8 @@ class Zend_Form_Decorator_HtmlTag extends Zend_Form_Decorator_Abstract
         foreach ((array) $attribs as $key => $val) {
             $key = htmlspecialchars($key, ENT_COMPAT, $enc);
             if (is_array($val)) {
-                if (array_key_exists('callback', $val)) {
+                if (array_key_exists('callback', $val)
+                    && is_callable($val['callback'])) {
                     $val = $val['callback']($this);
                 } else {
                     $val = implode(' ', $val);