Fix fatal error on reports

For some unknown reasons, some table reports end up with container fields
displayed as column. This leads to fatal errors as soon as an artifact
is displayed in this table.

Change-Id: Iec387606db4334af9e5d99f4a3d252e42c998377
Merge commit 'refs/changes/03/403/2' of http://gerrit.tuleap.net/tuleap into stable

* http://gerrit.tuleap.net/tuleap:
  Fix fatal error on reports
This is Tuleap 5.12.99.48