Fixes some overflows in the fixed-point pitch code

The overflows reported were:
- Syy in find_best_pitch() in pitch.c
- xy, xx and yy in remove_doubling() in pitch.c

The fixes are:
- Adaptive scaling in pitch_downsample() which also improves quality
- Adding a missing downshift in find_best_pitch()
- More conservative yshift when calling find_best_pitch()