Pending pledge payments logic may fail in contribution form

Pending pledge payments logic may fail in contribution form ...

... if there are several active pledges but only one of them has open items and it is not the last one checked ...

as the action code that is executed if $paymentsDue is set but $multpleDue is false
fetches values are taken from the most resent $payments result. This is "false" though
if the last pledge checked had no open items. The actual data for the open item
found is stored in $paymentsDue instead, so this variable needs to be used instead
of $payments