[giac] fix a/b/c -> a/(b*c)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@26126 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
[giac] wrap in caseval("...")

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@26127 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e