msm: camera: isp: Add null check for event callback function

This change is to handle race condition in which tasklet delayed
and continue execution after release context.

CRs-Fixed: 2667524
Change-Id: If52dbb72bbd85d83fe6eed2279c63072ae262d1b
Signed-off-by: Ayush Kumar <ayushkr@codeaurora.org>