Fix a few minor dissector bugs.

Change-Id: If9a2cb3456e6262c6c255f5b64cbad9e9c70038f
Reviewed-on: https://code.wireshark.org/review/3433
Petri-Dish: Bill Meier <wmeier@newsguy.com>
Reviewed-by: Bill Meier <wmeier@newsguy.com>