MLK-9760: ASoC: fsl_esai: fix NULL pointer issue in reset handler

When test with case arecord -Dhw:0,1 | aplay -Dhw:0,0, xrun happened,
the reset handler will be called, but for BE(backend) stream, the
substream->ops is null.
This patch is to fix this null pointer issue.

Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
(cherry picked from commit 4db112a8cd)

sound/soc/fsl/fsl_esai.c

@@ -775,7 +775,8 @@ static int stop_lock_stream(struct snd_pcm_substream *substream)
 {
 	if (substream) {
 		snd_pcm_stream_lock_irq(substream);
-		if (substream->runtime->status->state == SNDRV_PCM_STATE_RUNNING)
+		if (substream->runtime->status->state == SNDRV_PCM_STATE_RUNNING
+			&& substream->ops)
 			substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_STOP);
 	}
 	return 0;
@@ -784,7 +785,8 @@ static int stop_lock_stream(struct snd_pcm_substream *substream)
 static int start_unlock_stream(struct snd_pcm_substream *substream)
 {
 	if (substream) {
-		if (substream->runtime->status->state == SNDRV_PCM_STATE_RUNNING)
+		if (substream->runtime->status->state == SNDRV_PCM_STATE_RUNNING
+			&& substream->ops)
 			substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_START);
 		snd_pcm_stream_unlock_irq(substream);
 	}