Follow-up to r1648537:  Fix occasional segfault in DAG walk.

* subversion/libsvn_fs_fs/tree.c
  (open_path): Make sure our "HERE" node remains valid between iterations. 

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648591 13f79535-47bb-0310-9956-ffa450edef68