mod_dav_svn: Validate transaction names and activity ids.

* subversion/mod_dav_svn/repos.c
  (parse_vtxnstub_uri, parse_vtxnroot_uri): Validate transaction name.
  (prep_working, prep_activity): Validate activity id.

Patch by: philip


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1643437 13f79535-47bb-0310-9956-ffa450edef68