Changes == 0 to <= 0 for node id test

Changes == 0 to <= 0 for node id test
	* negative node-ids are special values that denote dummy nodes
	or errors, since valide nodes are positive, refusing the root
	with <= 0 rather than == 0 tells the compiler that the value
	of the id is (strictly) positive in the subsequent code,
	allowing the compiler to perform more agressive optimisations.

git-svn-id: svn+ssh://idea.nguyen.vg/svn/sxsi/trunk/bp@1219 3cdefd35-fc62-479d-8e8d-bae585ffb9ca