wp-commentsrss2.php produces invalid xml, because of a php notice error

[nbachiyski]

In case either there is no initialized $feed variable before including wp-commmentsrss2.php or the script is invoked direclty - an E_NOTICE error is generated, because $feed is accessed before it has been checked with isset().

If E_NOTICE is among the error_reporting levels and display_errors is on - the script procuces invalid xml file and most aggregators reject it.