NSS: Set packet length for initgroups
    
    Some groups could be skipped, but packet length was not trimmed.
    This is a reason why valgrind reported access to uninitialised bytes.
    Actually, it isn't a problem, because the first uint32 in body is number of
    sended gids.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2138