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.

trunk patch: http://svn.apache.org/r1485668
Submitted by: Ben Reser <ben reser.org>
Reviewed by: minfrin, covener, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1486461 13f79535-47bb-0310-9956-ffa450edef68