Fix dissection of GTP MM Context RAND field.

As suggested by Lükő Bálint to fix bug #9722.

Change-Id: Ie6b801a9660716c2c6efa6fd09b50a2481f5acd7
Reviewed-on: https://code.wireshark.org/review/110
Reviewed-by: Evan Huus <eapache@gmail.com>
Tested-by: Evan Huus <eapache@gmail.com>