audio: mux: Fix incorrect iterator in demux_trigger

demux_trigger() checks for cross-pipeline sinks to set the overrun_permitted
flag. Commit 96b4fdb ("buf: use API for iteration bsource_list in
components") incorrectly converted the bsink_list iteration to
comp_dev_for_each_producer instead of comp_dev_for_each_consumer.

Since demux has 1 input and multiple outputs, the loop must iterate over
consumers (sinks), not producers (sources). The incorrect iterator caused
the pipeline comparison to never match, making audio_stream_set_overrun()
unreachable.

Fix by using comp_dev_for_each_consumer with comp_buffer_get_sink_component
and add a defensive NULL check on the returned component pointer.

Issue found using semgrep with custom rules.

Signed-off-by: Tomasz Leman <tomasz.m.leman@intel.com>

Author: tmleman

src/audio/mux/mux.c

@@ -290,9 +290,11 @@ static int demux_trigger(struct processing_module *mod, int cmd)
 	 */
 	if (cmd == COMP_TRIGGER_PRE_START) {
 		struct comp_buffer *b;
+		struct comp_dev *sink_comp;
 
-		comp_dev_for_each_producer(mod->dev, b) {
-			if (comp_buffer_get_sink_component(b)->pipeline != mod->dev->pipeline)
+		comp_dev_for_each_consumer(mod->dev, b) {
+			sink_comp = comp_buffer_get_sink_component(b);
+			if (sink_comp && sink_comp->pipeline != mod->dev->pipeline)
 				audio_stream_set_overrun(&b->stream, true);
 		}
 	}