In mod_dav_svn, destroy an iterpool before returning error.

* subversion/mod_dav_svn/repos.c
  (do_walk): Destroy the iterpool created here before returning an error.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1643818 13f79535-47bb-0310-9956-ffa450edef68