ASoC: SOF: ipc: Add null pointer check for substream->runtime

When pcm stream is stopped "substream->runtime" pointer will be set
to NULL by ALSA core. In case host received an ipc msg from firmware
of type IPC_STREAM_POSITION after pcm stream is stopped, there will
be kernel NULL pointer exception in ipc_period_elapsed(). This patch
fixes it by adding NULL pointer check for "substream->runtime".
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Ajit Kumar Pandey <AjitKumar.Pandey@amd.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20211216232422.345164-3-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/ipc.c

@@ -623,7 +623,8 @@ static void ipc_period_elapsed(struct snd_sof_dev *sdev, u32 msg_id)
 
 	if (spcm->pcm.compress)
 		snd_sof_compr_fragment_elapsed(stream->cstream);
-	else if (!stream->substream->runtime->no_period_wakeup)
+	else if (stream->substream->runtime &&
+		 !stream->substream->runtime->no_period_wakeup)
 		/* only inform ALSA for period_wakeup mode */
 		snd_sof_pcm_period_elapsed(stream->substream);
 }