Follow-up to r1648591: Fix a similar segfault in the DAG walk code.

* subversion/libsvn_fs_fs/tree.c
  (open_path): We must always take the iteration start node HERE from
               the node path walked so far.  CHILD may live in ITERPOOL
               and must never be directly assigned to HERE anymore.
               This actually simplifies the iteration logic.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648612 13f79535-47bb-0310-9956-ffa450edef68