Added a guarantee that tracks won't be asked for their feature vector until

they are updated at least once with a detection.

dlib/svm/structural_track_association_trainer.h

@@ -222,6 +222,7 @@ namespace dlib
                     if (samples[i][j].size() > 0)
                     {
                         track_type new_track;
+                        new_track.update_track(samples[i][j][0].first);
                         typename track_type::feature_vector_type feats;
                         new_track.get_similarity_features(samples[i][j][0].first, feats);
                         return feats.size();

dlib/svm/track_association_function_abstract.h

@@ -58,6 +58,8 @@ namespace dlib
             feature_vector_type& feats
         ) const;
         /*!
+            requires
+                - update_track() has been called on this track at least once.
             ensures
                 - #feats == A feature vector that contains information describing how
                   likely it is that det is a detection from the object corresponding to