bind-9.16: isc_event_allocate cannot fail gracefully now
    
    isc_event_allocate internally uses isc_mem_get which cannot
    return NULL. So NULL-check is redundant.
    
    sync_barrierev_create and sync_finishev_create are always
    successful(except fatals which abort).
    
    https://gitlab.isc.org/isc-projects/bind9/-/commit/50e109d659