Use safer alternative to 'sprintf'.


git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1557 b0c55286-4f34-0410-a049-a1e7e93b0762
Correct a typo.


git-svn-id: https://fricas.svn.sourceforge.net/svnroot/fricas/trunk@1558 b0c55286-4f34-0410-a049-a1e7e93b0762