Fix possible uninitialized session key in netlogin auth

Bug:10312
Change-Id: Ia2be812e81b54d6df5a928312cca86ecd95aa6b2
Reviewed-on: https://code.wireshark.org/review/3236
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit 0aa4cb2d0eef6ab01109555fa5320486d13cc9c0)
Reviewed-on: https://code.wireshark.org/review/3237