diag: Fix diag crash due to memory corruption

Diag allocates memory based on the count received from userspace.
Add check to validate the memory pointer before derefencing it.

CRs-Fixed: 487981
Change-Id: Ibfee22d02bb2312192de7c811d9917ecab728269
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>