Significant spectral distortion in MC 160kbps test

commit sha: a34ef103 test: test_param_file_tests[Multi-channel 7_1_4 at 160 kbps, 48kHz in, 48kHz out, 7_1_4 out] Observations:

• Strong distortions were observed in channels 5 and above. Following is the plot for channel 5. Left axis is the spectrum of the floating-point output signal. Right axis is the spectrum of the deviation of the spectrum of the fixed-point output from that of the floating-point output.
• Similar issue was also observed with some time segments in MC bitrate switch tests.

Command lines:

DUT encoder command: ./IVAS_cod_ref -q -mc 7_1_4 160000 48 scripts/testv/stv714MC48c.wav tests/dut/param_file/enc/stv714MC48c.wav_Multi_channel_7_1_4_at_160_kbps_48kHz_in_48kHz_out_7_1_4_out.192 DUT decoder command: ./IVAS_dec -q -fr 20 7_1_4 48 tests/dut/param_file/enc/stv714MC48c.wav_Multi_channel_7_1_4_at_160_kbps_48kHz_in_48kHz_out_7_1_4_out.192 tests/dut/param_file/dec/stv714MC48c.wav_Multi_channel_7_1_4_at_160_kbps_48kHz_in_48kHz_out_7_1_4_out.dec.wav