mod_dav: When a PROPPATCH attempts to remove a non-existent dead

property on a resource for which there is no dead property in the same
namespace httpd segfaults. PR 52559

trunk patch: http://svn.apache.org/r1476642
Submitted by: Diego Santa Cruz <diego.santaCruz spinetix.com>
Reviewed by: minfrin, jim, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1486458 13f79535-47bb-0310-9956-ffa450edef68