Use packet_scope instead of a stack local

As clang pointed out we end up storing a reference to it in a global and (more
relevantly) pushing that global to a tap which would run after the current frame
has returned.

Thanks to Alexis for bringing this to my attention.

Change-Id: I3aac43a806d217b0dc8a973f6bb2fa48cdd041bb
Reviewed-on: https://code.wireshark.org/review/3289
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>