diag: Potential Null Pointer Dereference of user buf

Currently there is no check for the buf coming from
the user side,thus no way of knowing whether the buf
is null or not. This may lead to potential null
pointer dereference. This patch will fix that
possibility.

Change-Id: I9a66f3354c98bf03f9f5439a66078f247698d31c
CRs-fixed: 540570
Signed-off-by: Katish Paran <kparan@codeaurora.org>