Fix a segfault in our replay logic when presented with corrupted changed
path lists that try to add root nodes. 

* subversion/libsvn_repos/replay.c
  (path_driver_cb_func): We can never add a directory or file at "/".

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622944 13f79535-47bb-0310-9956-ffa450edef68