Fix for 3GPP issue 1798: Decoder crash for ParamUpmix MC 7.1+4 at 160kbps...

Fix for 3GPP issue 1798: Decoder crash for ParamUpmix MC 7.1+4 at 160kbps decoding to mono at 16/32kHz in ivas_ls_setup_conversion_process_mdct_fx()

Link #1798