SDAP: check that connection is open before bind
    
    Tivoli server does not return an empty response when being asked for the
    rootDSE data but an error. In this case the rootDSE lookup in SSSD will
    terminate the connection to the server and return a error. But since
    errors except timeouts are ignored SSSD will try to continue with the
    bind, but since the connection is already terminated this will fail as
    well. And this will terminate the whole operation.
    
    Make sure the connection is open before performing bind operation.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2435
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>