Fix dissection of 7 bits ASCII/GSM strings when the bit offset is not byte aligned

Bug: 10491
Change-Id: Ib55d83b7739050ba5afd84e8184af3c4608d5776
Reviewed-on: https://code.wireshark.org/review/4228
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>