BillingVariant: Fix CalculateModifiers() by restricting modifier year

Elwig/Helpers/Billing/BillingVariant.cs

@@ -168,7 +168,7 @@ namespace Elwig.Helpers.Billing {
                 SELECT d.year, d.did, d.dpnr, {AvNr}, 0, COALESCE(m.abs, 0), COALESCE(m.rel, 0)
                 FROM delivery_part d
                     LEFT JOIN delivery_part_modifier p ON (p.year, p.did, p.dpnr) = (d.year, d.did, d.dpnr)
-                    LEFT JOIN modifier m ON m.modid = p.modid
+                    LEFT JOIN modifier m ON (m.year, m.modid) = (d.year, p.modid)
                 WHERE d.year = {Year}
                 ON CONFLICT DO UPDATE
                 SET mod_abs = mod_abs + excluded.mod_abs,