NAS EPS: fix dissection of APN AMBR IE when first byte is set to 0xFE

Bug: 10699
Change-Id: I0f359639d5836f557370ab6d6aad45faa4417b6e
Reviewed-on: https://code.wireshark.org/review/5285
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>
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>