[task/config-class] Always specify the config table to use.

PHPBB3-9988

phpBB/includes/captcha/plugins/phpbb_captcha_gd_plugin.php

@@ -154,7 +154,7 @@ class phpbb_captcha_gd extends phpbb_default_captcha
 		$config_old = $config;
 
 		$config = new phpbb_config(array());
-		foreach ($old_config as $key => $value)
+		foreach ($config_old as $key => $value)
 		{
 			$config->set($key, $value);
 		}