Fix a crasher introduced on June 3.

* mod_dav.c (dav_method_propfind): the PROPFIND request might not have
    a body. Therefore, we cannot dereference 'doc' unconditionally.
    Test it to decide what to pass on to dav_begin_multistatus().

Submitted by: Ben Collins-Sussman <sussman@collab.net>,
              David Waite <mass@akuma.org>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100308 13f79535-47bb-0310-9956-ffa450edef68