Add paranoia check on the ra_svn receiver side.

* subversion/libsvn_ra_svn/client.c
  (ra_svn_get_dir): return an error when dirent names contain path separators

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1533812 13f79535-47bb-0310-9956-ffa450edef68