mod_dav: Avoid doing the walk on a COPY source for handling preconditions if

there are no preconditions provided.

* modules/dav/main/util.c:
  (dav_validate_request): avoid validating locks and ETags when there are
    no If headers providing them on a resource we aren't modifying.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1677462 13f79535-47bb-0310-9956-ffa450edef68