Fix: Overwrite bad value of limit payment date only if payment date is

defined.

htdocs/fourn/facture/fiche.php

@@ -221,7 +221,7 @@ elseif ($action == 'setdatef' && $user->rights->fournisseur->facture->creer)
 {
     $object->fetch($id);
     $object->date=dol_mktime(12,0,0,$_POST['datefmonth'],$_POST['datefday'],$_POST['datefyear']);
-    if ($object->date_echeance < $object->date) $object->date_echeance=$object->date;
+    if ($object->date_echeance && $object->date_echeance < $object->date) $object->date_echeance=$object->date;
     $result=$object->update($user);
     if ($result < 0) dol_print_error($db,$object->error);
 }