[distr] fixed an issue with casting AffLinDiscreteDistributions to LatticeDistributions (discovered by Kostas Oikonomou, ko@research.att.com )

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/distr@943 edb9625f-4e0d-4859-8d74-9fd3b1da38cb