audio: chain_dma: prevent NULL pointer dereference in chain_task_run

Validate cd, cd->dma_buffer, cd->chan_link, and cd->chan_host at the start
of chain_task_run() to handle stream teardown cleanly when buffers or channels
are freed before task completion.

Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>

Author: lrgirdwo

src/audio/chain_dma.c

@@ -168,6 +168,9 @@ static enum task_state chain_task_run(void *data)
 	uint32_t link_type;
 	int ret;
 
+	if (!cd || !cd->dma_buffer || !cd->chan_link || !cd->chan_host)
+		return SOF_TASK_STATE_COMPLETED;
+
 	/* Link DMA can return -EPIPE and current status if xrun occurs, then it is not critical
 	 * and flow shall continue. Other error values will be treated as critical.
 	 */