GTPv2: fix dissection of Triggering Events IE

Bug: 10470
Change-Id: Ieb9819e578f078e956861a4e2d7ab3d8b96416d6
Reviewed-on: https://code.wireshark.org/review/4130
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>