Pitch estimation pseudo-interpolation bug

[alebzk]

I might be wrong, but there could be a bug in the pitch pseudo-interpolation.
Luckily, it's not critical, but better solve if it's an actual bug.

At the end of pitch_search():

   if (best_pitch[0]>0 && best_pitch[0]<(max_pitch>>1)-1)
   {
      opus_val32 a, b, c;
      a = xcorr[best_pitch[0]-1];
      b = xcorr[best_pitch[0]];
      c = xcorr[best_pitch[0]+1];
      if ((c-a) > MULT16_32_Q15(QCONST16(.7f,15),b-a))
         offset = 1;
      else if ((a-c) > MULT16_32_Q15(QCONST16(.7f,15),b-c))
         offset = -1;
      else
         offset = 0;
   } else {
      offset = 0;
   }
   *pitch = 2*best_pitch[0]-offset;

I think that the last line should be *pitch = 2*best_pitch[0]+offset; (plus instead of minus).
For instance, when c-a > .7*(b-a) is true, it means that xcorr[best_pitch[0]+1] is the strongest correlation coefficient (a and b are almost the same, while c is greater than a). Hence, I would return 2*best_pitch[0]+1.

Note that at the end of remove_doubling() the same is done, but in fact *T0_ = 2 * T + offset; is assigned (which has a +). I haven't sent a pull request since this code comes from Opus (and it's used in other projects).

[alebzk]

Luckily, it's not a bug :) xcorr indices are "inverted lags" due to how indexing operates in the pitch buffer.