diag: Check buffer size against command structure size

Validate the buffer size against the parsing command structure size
before parsing to prevent possible out of bound error case.

CRs-Fixed: 2437341
Change-Id: I31c9a556539fce403691294a76160ae4936e7065
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>