Fix: Issue #396, when chroma features can not be computed

[Caparrini]

Due to issue #396

Chroma features can not be computed and there are a miss align between shapes in the assignation.

One of the test fails but I am not sure why:

def test_speaker_diarization():
        labels, purity_cluster_m, purity_speaker_m = \
            aS.speaker_diarization("test_data/diarizationExample.wav",
                                    4, plot_res=False)
>       assert purity_cluster_m > 0.9, "Diarization cluster purity is low"
E       AssertionError: Diarization cluster purity is low
E       assert 0.7714285714285715 > 0.9