Initialize whole buffer in GPRS Mobility and Session Management dissector

Conflicts:
	epan/dissectors/packet-gsm_a_gm.c

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
Reviewed-on: https://code.wireshark.org/review/3502
Reviewed-by: Balint Reczey <balint@balintreczey.hu>