small touch-ups

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@483 255f4249-2263-4417-b1ea-8498779e27a7
fixes to support plots of root node only

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@484 255f4249-2263-4417-b1ea-8498779e27a7
assure non-negative nreg

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/partykit@485 255f4249-2263-4417-b1ea-8498779e27a7