diag: Fix possible use-after-free issue for mdlog session info

Currently there is a possibility of accessing freed mdlog
session info and it's attributes after closing the session.
The patch adds protection while accessing mdlog session info
for preventing use-after-free issue.

CRs-Fixed: 2133028
Change-Id: Iba603bc2d75f84a604df2627e8fcb2a18acf4637
Signed-off-by: Hardik Arya <harya@codeaurora.org>