Handle incorrect sort key for forum index

When the sort key was manually altered, the forum index would crash

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs@20227 a0171fae-0946-0410-966d-b1775066cd26
Released 1.2.18, new version 1.2.19

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs@20228 a0171fae-0946-0410-966d-b1775066cd26
Added /rdf method refs #14360

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs@20234 a0171fae-0946-0410-966d-b1775066cd26
Added /rdf method refs #14360

git-svn-id: https://svn.eionet.europa.eu/repositories/Naaya/trunk/eggs@20235 a0171fae-0946-0410-966d-b1775066cd26