Bug fix for AudioEffectFormantShiftFD_OA_F32.h

[DD4WH]

In lines 126, 128, and 132, please do not use N_2, but N_2 -1
Otherwise the audio will contain nasty artefacts.
N_2 is FFT_Size / 2 + 1, but the index should only have a max of FFT_Size / 2

@chipaudette @boblark Keep up the good work!

[boblark]

Hi Frank - I saw that you were looking at frequency shifting of various sorts to help our ear's lack of high frequency response. An interesting topic and a good one for Chip, I suspect.

On the bug fix, this class is really Chip's. Chip, I can fix the constants as Frank found? Also, does this same issue show up in the Tympan library? Thanks, Frank, for finding this.

Bob

[DD4WH]

Hi Bob,
in the meantime, I have modified the file(s) a little more in my trial to improve the audio quality. And I found a few more places with the same issue.
Maybe it is more straightforward, if I try to submit a pull request.
I will try to do that, but it will take me a few days / weeks more. :-) git is not my closest friend . . .