audio: pipeline: guard against NULL source or sink in pipeline_copy()

When a pipeline is being stopped or unbound, the component unbind
handler may clear pipeline->source_comp before the low-latency
copy task finishes its final execution tick. Attempting to access
p->source_comp->direction without checking for NULL triggers a
synchronous PIF exception (DSP panic).

Add NULL checks for p->source_comp and start before proceeding
with the pipeline graph copy walk, returning 0 cleanly if the
pipeline endpoints are no longer valid.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

Author: singalsu

src/audio/pipeline/pipeline-stream.c

@@ -179,6 +179,11 @@ int pipeline_copy(struct pipeline *p)
 
 	PPL_LOCK(p->core);
 
+	if (!p->source_comp) {
+		PPL_UNLOCK();
+		return 0;
+	}
+
 	if (p->source_comp->direction == SOF_IPC_STREAM_PLAYBACK) {
 		dir = PPL_DIR_UPSTREAM;
 		start = p->sink_comp;
@@ -187,6 +192,11 @@ int pipeline_copy(struct pipeline *p)
 		start = p->source_comp;
 	}
 
+	if (!start) {
+		PPL_UNLOCK();
+		return 0;
+	}
+
 	data.start = start;
 	data.p = p;