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>