code coverage: ivas_param_mc_get_mono_stereo_mixing_matrices()

The code coverage reports that function ivas_param_mc_get_mono_stereo_mixing_matrices() is not currently executed- see https://ivas-codec-pc.3gpp.org/ivas-codec/coverage/lib_dec/ivas_mc_param_dec.c.gcov.html.

Is this function relevant or a new condition be added to the self-test to cover it?