mod_dav: Sending a MERGE request against a URI handled by mod_dav_svn with

the source href (sent as part of the request body as XML) pointing to a
URI that is not configured for DAV will trigger a segfault.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1485668 13f79535-47bb-0310-9956-ffa450edef68