Discontinuity for test case scaled +10dB for stereo mono output with DTX and PLC

This was first observed in the reference float code in the BASOP development repository and related to the issue Improper output observed for test case scaled +10dB input test_param_file_tests[stereo at 48 kbps, 48 kHz in, 48 kHz out, DTX on, MONO out, random FER at 5%].

Issue is still present in current version of main (f5c8b410)