GSM SMS: follow-up of gd65b7d5

Change-Id: I221e1411501e7c15eafe43e109f478684fa5ef10
Reviewed-on: https://code.wireshark.org/review/4408
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>