Event's null ethread pointer is sometimes dereferenced.

In this call: https://github.com/apache/trafficserver/blob/3a4c3dbbe340fc5e1c88c5375e5ed826ed9a797c/iocore/eventsystem/P_UnixEventProcessor.h#L123

the ethread field of the event structure is sometimes null (which causes a crash).

Here is an example stack dump:
```
[ 00 ] libtscore.so.9.1.10 ink_atomiclist_push ( ink_queue.cc:530 )
[ 01 ] traffic_server ProtectedQueue::enqueue(Event*) ( ProtectedQueue.cc:52 )
[ 02 ] traffic_server EventProcessor::schedule(Event*, int) ( P_UnixEventProcessor.h:123 )
[ 03 ] traffic_server TSContScheduleOnPool ( P_UnixEvent.h:31 )
[ 04 ] safetslua.so statsHandler ( ts_lua.c:215 )
[ 05 ] traffic_server INKContInternal::handle_event(int, void*) ( InkAPI.cc:1138 )
[ 06 ] traffic_server EThread::process_event(Event*, int) ( I_Continuation.h:219 )
[ 07 ] traffic_server EThread::execute_regular() ( UnixEThread.cc:273 )
[ 08 ] traffic_server execute ( UnixEThread.cc:364 )
[ 09 ] traffic_server EThread::execute() ( UnixEThread.cc:342 )
[ 10 ] traffic_server spawn_thread_internal ( Thread.cc:91 )
[ 11 ] libpthread-2.17.so start_thread ( pthread_create.c:307 )
```