sbus: register filter on new connection
    
    The filter is not again registered on new connection when the old connection
    was lost. This caused a segfault when the router is destroyed during shutdown.
    
    It also would not allow to recieve and process any messages as the filter
    function is needed for that. However, this was not very visible with
    current sssd architecture.
    
    Steps to reproduce:
    1. Run SSSD
    2. pkill sssd_be
    3. Wait for responders to reconnect to backend
    4. Shutdown SSSD
    5. It will crash without this patch
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3821
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>