added residCurvTest for glm.nb

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/car@396 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
trapped error resulting from groups= when n<3

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/car@397 edb9625f-4e0d-4859-8d74-9fd3b1da38cb