[new-owner-report] fix: payment->invoice gets invoice totals

previously the payment-amount deduction loop used the
payment-split-list to obtain the invoice-posting-split's amount. this
would occasionally fail and would return the invoice-payment-split
amount, obtaining the wrong sign.

modify to retrieve the invoice total via gncInvoice API.

therefore payment-amount, minus gncInvoiceGetTotal(inv) amounts,
results in the overpayment amount.

gnucash/report/business-reports/new-owner-report.scm

@@ -374,8 +374,7 @@
        (else
         (let* ((payment-split (car payment-splits))
                (inv (car payment-split))
-               (inv-split (cadr payment-split))
-               (inv-amount (AP-negate (xaccSplitGetAmount inv-split))))
+               (inv-amount (gncInvoiceGetTotal inv)))
           (lp (cdr payment-splits)
               (- amount inv-amount)
               (cons (list