event: Only process if event_init has been run.

Reported by @fourjay, a codepath that causes event_poll() to run before
event_init() will trigger a segfault as the events list will not have
been initialized. Exiting immediately from event_init() causes nvim to
hang, so just exit before running the events.

fixes #2339

src/nvim/os/event.c

@@ -124,7 +124,11 @@ void event_poll(int ms)
   }
 
   recursive--;  // Can re-enter uv_run now
-  process_events_from(immediate_events);
+
+  // In case this is run before event_init, don't process any events.
+  if (immediate_events) {
+    process_events_from(immediate_events);
+  }
 }
 
 bool event_has_deferred(void)