Check convex : add angles sum

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@32002 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e
#4111: fixed (was caused by NPE)

git-svn-id: http://dev.geogebra.org/svn/trunk/geogebra@32003 23ce0884-8a58-47d3-bc5c-ddf1cd5b9f9e