ASoC: dpcm: skip missing substream while applying symmetry

If for any reason, the backend does not have the requested substream
(like capture on a playback only backend), the BE will be skipped in
dpcm_be_dai_startup().

However, dpcm_apply_symmetry() does not skip those BE and will
dereference the be_substream (NULL) pointer anyway.

Like in dpcm_be_dai_startup(), just skip those BE.

Fixes: 906c7d69 ("ASoC: dpcm: Apply symmetry for DPCM")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-pcm.c

@@ -1911,10 +1911,15 @@ static int dpcm_apply_symmetry(struct snd_pcm_substream *fe_substream,
 		struct snd_soc_pcm_runtime *be = dpcm->be;
 		struct snd_pcm_substream *be_substream =
 			snd_soc_dpcm_get_substream(be, stream);
-		struct snd_soc_pcm_runtime *rtd = be_substream->private_data;
+		struct snd_soc_pcm_runtime *rtd;
 		struct snd_soc_dai *codec_dai;
 		int i;
 
+		/* A backend may not have the requested substream */
+		if (!be_substream)
+			continue;
+
+		rtd = be_substream->private_data;
 		if (rtd->dai_link->be_hw_params_fixup)
 			continue;