#506458 by quicksketch: Replace missing global user so that page caching works again.

includes/cache.inc

@@ -336,6 +336,8 @@ protected function garbageCollection() {
    *   valid item to load.
    */
   protected function prepareItem($cache) {
+    global $user;
+
     if (!isset($cache->data)) {
       return FALSE;
     }