Fix for issue 788: Distortion between 10k to 20k in fixed point OSBA Mono output

EVS 26.444 BE test passes.
pytests results: 597 failed, 381 skipped

MLD scores:
Original inputs:
MLD > 10: 64
MLD > 5 : 126
MLD < 5 : 407

+10dB inputs:
MLD > 10: 191
MLD > 5 : 198
MLD < 5 : 208

-10dB inputs:
MLD > 10: 23
MLD > 5 : 72
MLD < 5 : 502