mod_authn_socache.c: fix creation of default socache_instance.

In pre_config, default socache_provider is created, but socache_instance
initialization is missing. This leads to crash on startup if default
socache_provider is used (AuthnCacheSOCache is not called) and
AuthnCacheEnable or AuthnCacheProvideFor is used.

This problem has been introduced in r1531961.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1576233 13f79535-47bb-0310-9956-ffa450edef68