[rql2sql] fix potential crash when testing _q_invariant on a ColumnAlias instead of a Variable by always considering _q_invariant to false for them
"""server side objects
:organization: Logilab
:copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr
:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses
"""