#4076 make sure if CAS returns "?" function is undefined

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31695 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#4076 null pointer fix

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31696 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#4076 tidied code, added check for derivative

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31699 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
fix a problem in isEqual(segment): segments are equal even if their start and end points are swapped

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31700 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#4076 adding check to GeoFunctionNVar too

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@31701 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e