diag: Add NULL pointer checks for upd buffer

Currently there is a possibility of NULL pointer
dereference issue due to NULL pointers checks are
missing for upd buffer. The patch fixes the issue by
adding NULL pointer checks.

CRs-Fixed: 2146788
Change-Id: I3da40d9abd77e1825903c530f2311f9c2376fd5f
Signed-off-by: Hardik Arya <harya@codeaurora.org>