mod_dav: Do not segfault on PROPFIND with a zero length DBM. PR 52559

trunk patch: http://svn.apache.org/r1476645
Submitted by: Diego Santa Cruz <diego.santaCruz spinetix.com>
Reviewed by: minfrin, covener, sf


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1481513 13f79535-47bb-0310-9956-ffa450edef68