diag: Remove null pointer dereference from diag_md_session_close

Remove reference to session_info after handle is closed.
Store the peripheral masks in a local variable before closing the
session. Use stored value instead of closed session_info handle.

CRs-fixed: 930676
Change-Id: I08179b100926883f0cb84e0de35cc356e9cc9a03
Signed-off-by: Chris Lew <clew@codeaurora.org>