NPE fix

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@34733 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
Prevent Extremum from failing when CAS not initialized

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@34734 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e