Really fix use of external validators in listbox cells (for existing objects)

When I reverted [31533], I thought the 3rd replacement was useless:
I didn't notice [27255] was only for non-existing objects.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@31535 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Form/ListBox.py

@@ -2703,7 +2703,7 @@ class ListBoxValidator(Validator.Validator):
                     error_result_key = '%s_%s' % (my_field.id, o.uid)
                     key = 'field_' + error_result_key
                     try:
-                      value = my_field.validator.validate(my_field, key, REQUEST) # We need cell
+                      value = my_field._validate_helper(key, REQUEST) # We need cell
                       error_result[error_result_key] = value
                       if not result.has_key(o.getUrl()):
                         result[o.getUrl()] = {}