As noted in https://www.wireshark.org/lists/wireshark-dev/201604/msg00103.html
a protocol field isn't really a tv_buff, so allow for the possibility
of a NULL tv_buff in a "protocol type". If the tvb is NULL, use the
string of the protocol field for comparison
Bug: 12335
Change-Id: Ie12a5f7b31c7293c61006b0f70135d100a97c4e0
Reviewed-on: https://code.wireshark.org/review/15261
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>