viewer: don't update the length until the operation is done

Prevents the frame/audio caches from being truncated unnecessarily during
an operation that ultimately doesn't change the length.

app/node/output/viewer/viewer.cpp

@@ -200,6 +200,10 @@ void ViewerOutput::UpdateTrackCache()
 
 void ViewerOutput::VerifyLength()
 {
+  if (operation_stack_ != 0) {
+    return;
+  }
+
   NodeTraverser traverser;
 
   rational video_length;