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 behaviour of condition statements.

Change-Id: I461aafe20d641766dddec86d86793b1aa2f2ab6b
CRs-fixed: 549470
Signed-off-by: Katish Paran <kparan@codeaurora.org>