MDL-80818 QuickForm: Freeze all elements with the same name

There are some radio buttons in activity Completion Conditions settings
that must be frozen altogether, because all of them have the same name.

lib/pear/HTML/QuickForm.php

@@ -1571,15 +1571,16 @@ class HTML_QuickForm extends HTML_Common {
             $elementList = array_flip($elementList);
         }
 
+        $frozen = [];
         foreach (array_keys($this->_elements) as $key) {
             $name = $this->_elements[$key]->getName();
             if ($this->_freezeAll || isset($elementList[$name])) {
                 $this->_elements[$key]->freeze();
-                unset($elementList[$name]);
+                $frozen[$name] = true;
             }
         }
 
-        if (!empty($elementList)) {
+        if (count($elementList) != count($frozen)) {
             return self::raiseError(null, QUICKFORM_NONEXIST_ELEMENT, null, E_USER_WARNING, "Nonexistant element(s): '" . implode("', '", array_keys($elementList)) . "' in HTML_QuickForm::freeze()", 'HTML_QuickForm_Error', true);
         }
         return true;