diag: Add NULL pointer checks in mask updates

Currently, NULL pointer checks are missing in mask
update functions which might lead to NULL pointer
dereference issues. This patch fixes the issue by
adding appropriate checks.

CRs-Fixed: 2015104
Change-Id: I9df99208e283c8d90921c6e20a6a80f88c18a327
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>