Guard against buffer overflow when copying group key bytes into
security association structure.

Change-Id: I2f573a0626bab44aaca1ac6716bf5ac3d6a9b7ed
Reviewed-on: https://code.wireshark.org/review/3553
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>