diag: Null pointer check to avoid kernel panic

When accessing diag master table we need a check for
null pointer while iterating over the circular list.
Without this check there is a possibility of kernel panic.

CRs-Fixed: 1087738
Change-Id: I73ee708f2df6e1cd6b56fc40296ede490824f194
Signed-off-by: Gopikrishna Mogasati <gmogas@codeaurora.org>