GTPv2: further fixes for Trace Information IE dissection

Bug: 10470
Change-Id: I8b61e5601379a568da39cce2335122e91ab37ec4
Reviewed-on: https://code.wireshark.org/review/4142
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>