Fix recent breakage


git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1770 b0c55286-4f34-0410-a049-a1e7e93b0762
properly protect against failure

git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1771 b0c55286-4f34-0410-a049-a1e7e93b0762