Handle degenerate spantrees with two, one or zero points

(cherry picked from commit 8bc11fa94ca5ab17e007aaec30507505e377847c)

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/vegan@2917 edb9625f-4e0d-4859-8d74-9fd3b1da38cb