Fix: If end date is not defined, we can't say it is too late.

htdocs/fourn/class/fournisseur.facture.class.php

@@ -1378,7 +1378,7 @@ class FactureFournisseur extends CommonInvoice
             while ($obj=$this->db->fetch_object($resql))
             {
                 $this->nbtodo++;
-                if ($this->db->jdate($obj->datefin) < ($now - $conf->facture->fournisseur->warning_delay)) $this->nbtodolate++;
+                if (! empty($obj->datefin) && $this->db->jdate($obj->datefin) < ($now - $conf->facture->fournisseur->warning_delay)) $this->nbtodolate++;
             }
             $this->db->free($resql);
             return 1;