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>
    (cherry picked from commit fd60528321fd52720222ec35b895ade54cccb48d)