msm: camera2: Validate frame during timeout in cpp driver

During long stability stress runs, invalid frames are encountered
in timeout processing. When frame acknowledgement is delayed by
firmware due to various reasons the timeout handler could hit
frame that are already processed. Hence adding NULL condition
check to avoid invalid frame access.

Change-Id: Icaa5e630d5190a7bf0a4e07a95d08e116cb304db
Signed-off-by: Rajakumar Govindaram <rajakuma@codeaurora.org>