BillingVariant: Fix retrieval of last used payment variant

Elwig/Helpers/Billing/BillingVariant.cs

@@ -40,7 +40,7 @@ namespace Elwig.Helpers.Billing {
                         LEFT JOIN payment_variant l ON l.year = s.year
                             AND l.avnr = (SELECT avnr
                                           FROM payment_variant
-                                          WHERE NOT test_variant
+                                          WHERE year = s.year AND NOT test_variant
                                           ORDER BY COALESCE(transfer_date, date) DESC, avnr DESC
                                           LIMIT 1)
                         LEFT JOIN (SELECT year, MAX(tgnr) AS tgnr FROM credit GROUP BY year) t ON t.year = s.year