Initialize whole buffer in GPRS Mobility and Session Management dissector

Bug: 10216
Change-Id: I572a7a6ce0f816063f02397b667dd46c990cf73e
Reviewed-on: https://code.wireshark.org/review/2583
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
(cherry picked from commit dd7134d907350ccc574cdec596f4162860912bb9)
Reviewed-on: https://code.wireshark.org/review/2585
(cherry picked from commit abae269b462b0b39fda7a21b24a53d0033662662)
Reviewed-on: https://code.wireshark.org/review/3436
Reviewed-by: Balint Reczey <balint@balintreczey.hu>