updated comparator to be consistent with equals

git-svn-id: svn://svn.forge.objectweb.org/svnroot/sat4j/maven/trunk@2386 ba638df5-4473-46d1-82f8-c3ae2a17a6e1
Fixed a bug that made sat4j-pb crash while using cardinality constraints detection.

git-svn-id: svn://svn.forge.objectweb.org/svnroot/sat4j/maven/trunk@2387 ba638df5-4473-46d1-82f8-c3ae2a17a6e1