ses: initialize pres_ctx_id in session struct

Bug: 10454
Change-Id: Ib381646cad0a039514117735c48b461c9950f705
Reviewed-on: https://code.wireshark.org/review/4033
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit 433a444d148f86f2562f804d25a57d00dc277cc0)
Reviewed-on: https://code.wireshark.org/review/4036