mixin-mixout: handle congestion corner case without assert

If one mixout (A) is not able to process data, the 'sinks_free_frames'
will be truncated compared other mixout buffers. When the mixin process
is producing data to another mixout buffer (B), the number of already
mixed frames may be larger than 'sinks_free_frames'. Further it is
possible this mixout (B) has pending frames, leading to a larger
'start_frame' value.

This is a valid and possible scenario, but will trigger
  assert(sinks_free_frames >= start_frame)
in current code.

As 'frames_to_copy' is already correctly calculated as limited to
available free frames across all mixouts, no extra logic is needed when
this condition happens. Mixin is not able to proceed until the congested
mixout (A) has room again for further samples.

Link: #6896
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

src/audio/mixin_mixout/mixin_mixout.c

@@ -423,7 +423,6 @@ static int mixin_process(struct processing_module *mod,
 		 * performance problems with processing data on time in mixout.
 		 */
 		start_frame = mixout_data->pending_frames[source_index];
-		assert(sinks_free_frames >= start_frame);
 
 		sink_c = buffer_acquire(sink);