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

Submitted by: Diego Santa Cruz <diego.santaCruz spinetix.com>
Tested by William Lewis <wiml omnigroup com>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1476645 13f79535-47bb-0310-9956-ffa450edef68