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