fixed bug described by Lennart in mail "Re: [GenABEL-dev] [Genabel-commits] r1596 - branches/ProbABEL-0.50" which was introduced with revision 1596

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/genabel@1598 edb9625f-4e0d-4859-8d74-9fd3b1da38cb