crush: fix memory leak when destroying tree buckets

Reflects ceph.git commit 46d63d98434b3bc9dad2fc9ab23cbaedc3bcb0e4.

Reported-by: Alexander Lyakas <alex.bolshoy@gmail.com>
Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>