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

Also, cleaned up a unnecessary global variable.

Bug: 10611

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>

Conflicts:
 epan/dissectors/packet-ptp.c

Change-Id: Ifb9bcde70405d591c21bd4e10195e1b192cba98b
Reviewed-on: https://code.wireshark.org/review/5062
Reviewed-by: Michael Mann <mmann78@netscape.net>