MDL-31006 fix various notices in PHP54

PHP54 compatibility

blocks/navigation/block_navigation.php

@@ -195,6 +195,7 @@ class block_navigation extends block_base {
         
         // Grab the items to display
         $renderer = $this->page->get_renderer('block_navigation');
+        $this->content = new stdClass();
         $this->content->text = $renderer->navigation_tree($navigation, $expansionlimit, $options);
 
         // Set content generated to true so that we know it has been done