Don't use undefined map[index] = map.size()

* The LHS map[index] can get executed first before map.size() and we end
  up with indices that are 1 higher than we expect

qrenderdoc/Windows/PerformanceCounterViewer.cpp

@@ -230,7 +230,7 @@ void PerformanceCounterViewer::CaptureCounters()
       {
         if(eventIdToRow.contains(result.eventId))
           continue;
-        eventIdToRow[result.eventId] = eventIdToRow.size();
+        eventIdToRow.insert(result.eventId, eventIdToRow.size());
       }
 
       ui->counterResults->setColumnCount(headers.size());