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

PR: 52559
Backports: r1476645
Submitted by: Diego Santa Cruz <diego.santaCruz spinetix.com>
Reviewed by: rjung, wrowe, trawick



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497463 13f79535-47bb-0310-9956-ffa450edef68