Error messages on contribute page submit for IPN style processors (with no payment fields on the form)

NOTE: This was affecting Google Checkout and any other IPN processor where paymentFields are not collected on the form.
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On a contribution page on 4.2 using the Payment Express processor I get the message

warning: Invalid argument supplied for foreach() in /var/www/civicrm/civicrm-fuzion-d6-4.2/CRM/Contribute/Form/Contribution/Main.php on line 1103.

The line is

foreach ($self->_paymentFields as $name => $fld) {

_paymentFields isn't declared on the CRM_Contribute_Form_Contribution_Main class & I'm not sure where it should be set.

Probably it's fine just to wrap in an

if(isset($self->_paymentFields)){
}