Merge r589177 from trunk:

Fix possible crash at startup in case of nonexistent DocumentRoot.
PR#39722

Submitted by: niq
Reviewed by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@589618 13f79535-47bb-0310-9956-ffa450edef68