ASoC: SOF: fix an Oops, caused by invalid topology

It is possible to create invalid topology that will cause a kernel
Oops when trying to allocate buffers for a NULL substream.
Specifically such an Oops was caused by a topology, where a DAI on a
capture pipeline was referencing the PCM ID from a playback pipeline.
Fix the Oops by explicitly checking for NULL.
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200124213625.30186-2-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/pcm.c

@@ -591,6 +591,11 @@ static int sof_pcm_new(struct snd_soc_component *component,
 		"spcm: allocate %s playback DMA buffer size 0x%x max 0x%x\n",
 		caps->name, caps->buffer_size_min, caps->buffer_size_max);
 
+	if (!pcm->streams[stream].substream) {
+		dev_err(component->dev, "error: NULL playback substream!\n");
+		return -EINVAL;
+	}
+
 	snd_pcm_set_managed_buffer(pcm->streams[stream].substream,
 				   SNDRV_DMA_TYPE_DEV_SG, sdev->dev,
 				   le32_to_cpu(caps->buffer_size_min),
@@ -609,6 +614,11 @@ static int sof_pcm_new(struct snd_soc_component *component,
 		"spcm: allocate %s capture DMA buffer size 0x%x max 0x%x\n",
 		caps->name, caps->buffer_size_min, caps->buffer_size_max);
 
+	if (!pcm->streams[stream].substream) {
+		dev_err(component->dev, "error: NULL capture substream!\n");
+		return -EINVAL;
+	}
+
 	snd_pcm_set_managed_buffer(pcm->streams[stream].substream,
 				   SNDRV_DMA_TYPE_DEV_SG, sdev->dev,
 				   le32_to_cpu(caps->buffer_size_min),