MCT: missing zeroing-out of buffer when channel mode is MCT_CHAN_MODE_IGNORE

For MCT channels with zero content, the respective channel is ignored completely via a flag. In the decoder, when mct_chan_mode is MCT_CHAN_MODE_IGNORE, the IMDCT is skipped and output buffers are set to zero directly, skipping all calculations and the overlap-add-process. The buffer st->hHQ_core->old_out is used in the IMDCT to provide parts of the last frame's output for the OLA. This buffer needs to be set to zero for ignored frames as well. Otherwise, signal bursts can happen after the ignore period (currently only observed together with PLC).

This is the cause of this issue in BASOP: sa4/audio/ivas-basop#903 and a blocker for its resolution.

Example (from basop issue):

Example with zeroing of mentioned buffer: