Permalinks can be created which point to a nonexistent build symlink

A user reported a situation with a project which had a number of builds, none of which had associated numeric symlinks because the master was running on Windows at the time. Then the master was switched to Unix. Jenkins created a lastSuccessfulBuild symlink, but it was pointing to a nonexistent numeric symlink. It should either avoid creating the permalink in this case, or first create the numeric symlink.