`ProcessorTrackerLandmark::processNew: Replaced empty list assert statements by clearing the corresponding lists

In ProcessorTrackerLandmark::processNew, 3 lists were recommended to be empty: new_features_last_
new_features_incoming_
new_landmarks_

Some recent changes caused the list not to be empty anymore at this point in some cases. If someone has precisions regarding these two "some", please write it in the comments. Also, maybe clearing the lists in ProcessNew is only healing the symptoms and other changes should be made.