Announce messages should only have a TLV if the message length is > 64.

Also, cleaned up a unnecessary global variable.

Bug: 10611
Change-Id: I8ef5658eeb1f7a240b508833baac64719af144d8
Reviewed-on: https://code.wireshark.org/review/5029
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>