ASoC: soc-pcm: Add NULL check in BE reparenting

Add NULL check in dpcm_be_reparent API, to handle
kernel NULL pointer dereference error.
The issue occurred in fuzzing test.

Signed-off-by: Srinivasa Rao Mandadapu <quic_srivasam@quicinc.com>
Link: https://lore.kernel.org/r/1669098673-29703-1-git-send-email-quic_srivasam@quicinc.com
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/soc-pcm.c

@@ -1247,6 +1247,8 @@ static void dpcm_be_reparent(struct snd_soc_pcm_runtime *fe,
 		return;
 
 	be_substream = snd_soc_dpcm_get_substream(be, stream);
+	if (!be_substream)
+		return;
 
 	for_each_dpcm_fe(be, stream, dpcm) {
 		if (dpcm->fe == fe)