[giac] make sure z((1,2)) gives 0

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25787 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
[giac] typo

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@25788 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e