nss: Do not ignore default vaue of SYSDB_INITGR_EXPIRE
When SYSDB_INITGR_EXPIRE had default value (0) then value of
SYSDB_CACHE_EXPIRE was used as initgroups expire attribute.
The right apoach is already used in responder_cache_req.c
Reviewed-by: Pavel Březina <pbrezina@redhat.com>