RESPONDER: Fix segfault in sss_packet_send()
    
    There are several places (all error-handling) where sss_cmd_done()
    is called with no response packet created. As a short-term
    solution, we need to check whether the packet is NULL and simply
    return EINVAL. client_send() (the consumer) will then forcibly
    disconnect the client (which will return PAM_SYSTEM_ERR to the
    client).