[ticket/14278] Check if user_id is set and fall back to ANONYMOUS

PHPBB3-14278

phpBB/phpbb/install/module/update_database/task/update.php

@@ -172,7 +172,7 @@ class update extends task_base
 		{
 			$this->log->add(
 				'admin',
-				$this->user->data['user_id'],
+				(isset($this->user->data['user_id'])) ? $this->user->data['user_id'] : ANONYMOUS,
 				$this->user->ip,
 				'LOG_UPDATE_DATABASE',
 				false,