smb2: fully initialize dummy session

A few fields weren't being zeroed and were causing valgrind warnings.

Bug: 10569
Change-Id: I30e1e70c9445376806f385f3578b65bf6e320f8b
Reviewed-on: https://code.wireshark.org/review/4703
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit d07c65de126f976190f1ce8d07ed8fe7deb90696)
Conflicts: epan/dissectors/packet-smb2.c
Reviewed-on: https://code.wireshark.org/review/5178
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>