mod_dav_svn: Deal with some edge cases on producing the bogus path for r->filename.

* subversion/mod_dav_svn/mod_dav_svn.c
  (dav_svn__translate_name): dav_svn_split_uri() doesn't actually reliably give
    you an fspath but sometimes it's just a relpath (difference being leading /).
    Workaround this in a backportable way.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537700 13f79535-47bb-0310-9956-ffa450edef68