mod_dav_svn: Fix crashes with other modules that don't expect r->filename
to be NULL (e.g. mod_wsgi).

* subversion/mod_dav_svn/mod_dav_svn.c
  (dav_svn__translate_name): Calculate a bogus path to put in r->filename
    of the form "svn:/path/to/repo/path/in/repo".


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1537360 13f79535-47bb-0310-9956-ffa450edef68