Fix ProbABEL bug #5404 "ProbABEL's R check for Cox regression doesn't check if the survival package is installed".

Solution: simply quit R if the survival package is not installed.


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/genabel@1652 edb9625f-4e0d-4859-8d74-9fd3b1da38cb