BACAPP: fix dissection of event parameters for event notifications of Event_Type Change_of_Characterstring

Implementation for this event parameter type was missing

Bug: 9625
Change-Id: I575d387ec4764fa1ecd365624d13e3f41e2473c6
Reviewed-on: https://code.wireshark.org/review/5189
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>