ASoC: Tegra: tegra_pcm_deallocate_dma_buffer: Don't OOPS

Not all PCM devices have all sub-streams. Specifically, the SPDIF driver
only supports playback and hence has no capture substream. Check whether
a substream exists before dereferencing it, when de-allocating DMA
buffers in tegra_pcm_deallocate_dma_buffer.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org

sound/soc/tegra/tegra_pcm.c

@@ -309,9 +309,14 @@ static int tegra_pcm_preallocate_dma_buffer(struct snd_pcm *pcm, int stream)
 
 static void tegra_pcm_deallocate_dma_buffer(struct snd_pcm *pcm, int stream)
 {
-	struct snd_pcm_substream *substream = pcm->streams[stream].substream;
-	struct snd_dma_buffer *buf = &substream->dma_buffer;
+	struct snd_pcm_substream *substream;
+	struct snd_dma_buffer *buf;
+
+	substream = pcm->streams[stream].substream;
+	if (!substream)
+		return;
 
+	buf = &substream->dma_buffer;
 	if (!buf->area)
 		return;