use voucher ID for filename, as tracking ID might not be present

models/dp_delivery_carrier.py

@@ -92,7 +92,7 @@ class DPDeliveryCarrier(models.Model):
         im.add_position(position)
         r = im.checkoutPNG()
         voucher = r.shoppingCart.voucherList.voucher[0]
-        filename = 'DP'+voucher.trackId+'.png'
+        filename = 'DP'+voucher.voucherId+'.png'
         tracking_nr = ' '
         if voucher.trackId:
             tracking_nr += voucher.trackId