Expected behaviour

If no valid path could be decoded using the Viterbi algorithm, an empty array should be returned.

Actual behaviour

If self.correct == True, an IndexError is thrown in line 301 of downbeats.py, due to beat_range being empty.

Steps needed to reproduce the behaviour

import madmom
import numpy as np
import scipy

beat_tracker = madmom.features.downbeats.DBNDownBeatTrackingProcessor(fps=100.0, correct=True, beats_per_bar=4)
beat_activation = np.random.uniform(0, 1, 100)
downbeat_activation = np.random.uniform(0, 1, 100)

beat_decoder.process(np.dstack((beat_activation, downbeat_activation))[0])

Information about installed software

Please provide some information about installed software.

madmom.version == '0.16.1'
np.version == '1.19.5'
scipy.version == '1.7.2'