diag: Safeguard for bound checks and integer underflow

At certain point in diag driver there can be integer underflow
and thus can lead to memory leak. Bound checks are placed to
ensure correct behavior of condition statements.

Change-Id: I47e02f764c2c7412db6f90fd42192fee32a761d3
CRs-fixed: 549470
Signed-off-by: Katish Paran <kparan@codeaurora.org>