SHOW PROCESSLIST.

The problem was a race condition: if the Event Scheduler was not
quick enough, the following scenario happens:
  - The Event Scheduler picks up the created event;
  - The event is executed;
  - event_scheduler_thread->proc_info is set to NULL;
  - The client issues SELECT FROM I_S.

The fix is to wait for the Event Scheduler to reach 'Waiting
for next activation' state.