Ensure dissector data parameter is used instead of pinfo->private_data when passing data between dissectors using the "media_type" subdissector.

Change-Id: I82cbfed770b41404bc42cb6a4413db07d04dffdc
Reviewed-on: https://code.wireshark.org/review/5462
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>