backport r678949 from trunk:

propogate a NULL get_realm_hash() implementation from AuthnProviderAlias back
to mod_auth_digest.  PR 45196

Submitted by: covener
Reviewed by: rpluem, jerenkrantz



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@696364 13f79535-47bb-0310-9956-ffa450edef68