Bug fix to binary trees variant

David pointed out an obvious error in this variant that I just
committed: I tried to free the tree node after returning...
Whoops!  On the plus side, it would've been super fast probably!

This commit fixes that error (with any luck).



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21984 3a8e244f-b0f2-452b-bcba-4c88e055c3ca