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

[ Upstream commit db8f91d4

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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>
Signed-off-by: Sasha Levin <sashal@kernel.org>