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

[x] Updates made in TCX noise filling and IGF dec with which distortions are reduced.

EVS 26.444 BE test passes.
decoder pytests:
MLD scores:
Original inputs: 560 failed, 374 skipped
MLD > 10: 55
MLD > 5 : 125
MLD < 5 : 380
+10dB inputs: 560 failed, 374 skipped
MLD > 10: 184
MLD > 5 : 178
MLD < 5 : 198
-10dB inputs: 560 failed, 374 skipped
MLD > 10: 13
MLD > 5 : 62
MLD < 5 : 485

encoder pytests results:
MLD scores:
Original inputs: 560 tests: 235 passes, 325 failures, 374 skipped
MLD > 10: 16
MLD > 5 : 24
MLD < 5 : 520
+10dB inputs: 560 tests: 260 passes, 300 failures, 374 skipped
MLD > 10: 24
MLD > 5 : 47
MLD < 5 : 489
-10dB inputs: 560 tests: 252 passes, 308 failures, 374 skipped
MLD > 10: 5
MLD > 5 : 18
MLD < 5 : 537