Remove get_next_di() and just use packet-scope for those

Presumably that was added for tap purposes, but packet-scope is much simpler and
less dangerous. Noticed while investigating the scan-build issues with
stack-local variables being pointed to by globals.

Change-Id: I851d756b103df71079b656e624f7472354c15862
Reviewed-on: https://code.wireshark.org/review/3290
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Petri-Dish: Anders Broman <a.broman58@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>