diag: Set buffer to NULL after freeing from mempool

Currently we are not marking buffer as NULL after
freeing from mempool, which can cause to double free
when multiple threads are trying to free same mempool
buffer. The patch marks buffer as NULL after freeing.

Change-Id: I47129e38eb68bd50a262a791340add524db80209
Signed-off-by: Hardik Arya <harya@codeaurora.org>