mod_authn_socache: Fix creation of default socache_instance. Fixes crash

                   on startup if default socache_provider is used and
                   AuthnCacheEnable or AuthnCacheProvideFor is used.
                   This problem has been introduced in r1531961.
                   PR 56371.

Submitted by: jkaluza
Reviewed by: ylavic, jim, trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1592991 13f79535-47bb-0310-9956-ffa450edef68