diag: Add proper checks to fix possible out-of-bound issue

Currently, there is a possibility of out-of-bound access while
processing data received from user space. The patch adds proper
checks for valid address.

CRs-Fixed: 2048536
Change-Id: I1e0fc7a9d69e69f3326429d5d9540dd3bb1a59b0
Signed-off-by: Hardik Arya <harya@codeaurora.org>