ASoC: wcd_cpe_services: Fix possible NULL pointer de-reference

Fix the cpe_svc_core_cmi_handler function to perform NULL check before
accessing the parameter provided to avoid possible NULL pointer
de-reference.

CRs-fixed: 925138
Change-Id: I2a2b9b2c30bd94d97aa6ab89fa1db59650025d2f
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>