msm: camera: Check for valid mem object before using it.

Add checks for validity of the mem objects for the buffer
planes before using them, to prevent any potential crashes.

Change-Id: I1221092f8c13912479c72c2bd8385b93fd7226b0
Signed-off-by: Azam Sadiq Pasha Kapatrala Syed <akapatra@codeaurora.org>