diag: Mark Buffer as NULL after freeing

There is a possibility of use-after-free and
double free because of not marking buffer as
NULL after freeing. The patch marks buffer
as NULL after freeing in error case.

Change-Id: Iacf8f8a4a4e644f48c87d5445ccd594766f2e156
Signed-off-by: Hardik Arya <harya@codeaurora.org>