diag: Mark global buffer NULL after freeing from mempool

Currently we are not marking buffer as NULL after freeing
from mempool which is causing double free of data buffer.
The patch marks global hdlc and non_hdlc data buffer as
NULL after freeing it from mempool.

Change-Id: I5f389cfbb981a6e5d0c880c2112f7e4cf2cae433
Signed-off-by: Hardik Arya <harya@codeaurora.org>