Do not use packet scoped memory in GUI

It will trigger an assertion

Change-Id: I2436c11b45e1505a94256a06ed3ad0c5480e034b
Reviewed-on: https://code.wireshark.org/review/5953
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>