diag: Add NULL pointer checks

Currently, there is a possibility of NULL pointer dereference
while accessing diag master table. The patch adds proper check
for null pointer while iterating over the list.

CRs-Fixed: 2077525
Change-Id: I51075b7a7f3acce0cb27822ad1acd8a5894cdaa9
Signed-off-by: Hardik Arya <harya@codeaurora.org>