ANSI IS-637-A: fix dissection of fragmented Unicode SMS

The computation of the number of characters available was wrong when the User Data Header is present

Change-Id: I2809c1460316530654a997b26cfc33c60a32fd6f
Reviewed-on: https://code.wireshark.org/review/4284
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>
(cherry picked from commit e520b2b80002de80cf28e62abb537873d2abd259)
Reviewed-on: https://code.wireshark.org/review/4286