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>