msm: camera: cdm: Fix dangling pointer issue

Explicitly set pointer to NULL after freeing the
memory. This avoid causing memory corruption incase
the memory is already freed.

CRs-Fixed: 2631601
Change-Id: I2f4db99ccfefb2516e98ea5150a2bd5a342819e5
Signed-off-by: Alok Chauhan <alokc@codeaurora.org>