Allow all zero rows in mel computation matrix.

[csukuangfj]

import kaldifeat

opts = kaldifeat.FbankOptions()
opts.frame_opts.samp_freq = 8000
opts.mel_opts.num_bins = 128

fbank = kaldifeat.Fbank(opts)

throws the following error:

x: first_index != -1 && last_index >= first_index && "You may have set num_mel_bins too large."

However, in lhotse, the following code runs correctly

from lhotse import *

extractor = Fbank(FbankConfig(num_mel_bins=128, sampling_rate=8000))

And if we use

We can see that it allows rows to contain all zeros in the matrix.

This PR updates the C++ code in kaldifeat to match that.