diag: Fix race condition while accessing mdlog session info

This fix allows to avoid possible race condition while
accessing mdlog session info. Without this check there
is a possibility of poison overwritten occurrence.
Also this fix allows mdlog client to check the validity
of process descriptor. Without this fix there is
possibility of crash.

CRs-Fixed: 2022664
Change-Id: Ia10e83c7376de5cab92dd2ec4ec4aa8074223ff3
Signed-off-by: Gopikrishna Mogasati <gmogas@codeaurora.org>