diag: Validate command length against size of command structure

While processing a packet containing command request, buffer size
need to be checked against size of the command structures that is
being parsed to prevent possible out of bound access.

CRs-Fixed: 2432633
Change-Id: I048bdbd0c096a6d03501bdd5b1d2d4bb50d45dd6
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>