renderer: prevent infinite loop occurring on some time invalidations

app/render/backend/renderbackend.cpp

@@ -148,6 +148,10 @@ void RenderBackend::SetViewerNode(ViewerOutput *viewer_node)
     copied_viewer_node_ = static_cast<ViewerOutput*>(viewer_node_->copy());
     copy_map_.insert(viewer_node_, copied_viewer_node_);
 
+    // We begin an operation and never end it which prevents the copy from unnecessarily
+    // invalidating its own cache
+    copied_viewer_node_->BeginOperation();
+
     NodeGraphChanged(viewer_node_->texture_input());
     NodeGraphChanged(viewer_node_->samples_input());
     ProcessUpdateQueue();

app/render/framehashcache.cpp

@@ -147,6 +147,9 @@ QString FrameHashCache::GetFormatExtension()
 
 QVector<rational> FrameHashCache::GetFrameListFromTimeRange(TimeRangeList range_list, const rational &timebase)
 {
+  // If timebase is null, this will be an infinite loop
+  Q_ASSERT(!timebase.isNull());
+
   QVector<rational> times;
 
   while (!range_list.isEmpty()) {