Remove unnecessary memory reallocation in fft (whisper/2080)

Remove unnecessary memory reallocation in fft (whisper/2080)

fft_out needs to be twice the frame_size, not the frame_step.  It is resized in fft() anyway, but this change prevents an unnecessary reallocation.

n_fft must match the mel filter size, so it is best not to calculate it from the framesize.

We only need to get the magnitudes for half the spectrum since the other half is a mirror and not used in the mel filter loop later.