Fixed bug in platinum that was causing segfault

I finally found the bug that was causing segfaults. In `platinum`, the residual variable has `f0_length` rows and `fft_size+1` columns, but you allocated only `fft_size/2+1` columns (this is the right amount of columns for the spectrogram, but not for the residual).

This works for me on OS X but I am going to test on Linux as well.