Fix "Audio buffer is not finite everywhere" by force initializing buffer for spec_s

[lspr98]

Initializing spec_s with np.ndarray can contain Nan / Inf according to the numpy docs, since it does not overwrite the underlying buffer memory. This causes the "Audio buffer is not finite everywhere" error in cmb_spectrogram_to_wave, as already reported in multiple issues (#1021, #1045, #1147, #1153, #1262, to name a few). Initializing with np.zeros populates the buffer with zeros, avoiding the issue.