message: Fix segfault in EventSpan::Update.
diff: @@ -119,6 +119,7 @@ std::vector<EventTypeSpan> ToNonOverlappedEvents(
  std::vector<EventBoundary> event_boundaries =
  GenerateEventBoundaries(overlapped_events);
  std::vector<EventTypeSpan> result;
  + if (event_boundaries.empty()) return result;
  result.reserve(event_boundaries.size());
  PriorityTracker priority_tracker;
  for (int64 i = 0; i < (event_boundaries.size() - 1); i++) {
  