Fix for issue 754: Concealment artifacts at 48kbps stereo

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

MLD scores:
Original inputs:
MLD > 10: 62
MLD > 5 : 134
MLD < 5 : 401

+10dB inputs:
MLD > 10: 193
MLD > 5 : 198
MLD < 5 : 206

-10dB inputs:
MLD > 10: 23
MLD > 5 : 75
MLD < 5 : 499