msm: ocmem: Check for NULL pointer from get_zone

The get_zone function might return NULL if it is
unable to find the zone for the associated client
id. Add check to prevent NULL pointer dereferencing.

Change-Id: Icfe5ebce03e05a5592042fdf1be747ea7f93ba67
Signed-off-by: Neeti Desai <neetid@codeaurora.org>