Fix for issue 769: Instability in stereo synthesis at 48 kbps

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

MLD scores:
Original inputs:
MLD > 10: 64
MLD > 5 : 131
MLD < 5 : 402

+10dB inputs:
MLD > 10: 194
MLD > 5 : 198
MLD < 5 : 205

-10dB inputs:
MLD > 10: 24
MLD > 5 : 75
MLD < 5 : 498