Fixing a bug in add_ff_action_public_fields(), by removing the start argument and defining it inside the function.

Change-Id: I6205384b3bcf38128912baf413b4455357d25347
Reviewed-on: https://code.wireshark.org/review/3184
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit 0501465a3acb2abc108177ea5149ec8de4a05210)
Reviewed-on: https://code.wireshark.org/review/3361
Reviewed-by: Anders Broman <a.broman58@gmail.com>