diag: Fix possible buffer corruption by proper typecasting

This patch removes the possibilty of header length and read length
having negative values and thereby possible buffer corruption.

CRs-Fixed: 1002389
Change-Id: Id74650d5c4aaf9f84a56372f60ff5a40374e8f7b
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>