split_skeleton: check that the treenode is not root.
split_skeleton: check that the treenode is not root.

htem/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,fzadow/CATMAID