mip: fix (what appear to be) bugs in CVSE GRE key extension dissection

Change-Id: I43dc48de62eda3a62bef4bf4a40fd18cb45751f3
Reviewed-on: https://code.wireshark.org/review/5733
Reviewed-by: Anders Broman <a.broman58@gmail.com>
(cherry picked from commit 8ccdc07e8e1824398b63d2478958e38ba543e88b)
Reviewed-on: https://code.wireshark.org/review/5837
Reviewed-by: Bill Meier <wmeier@newsguy.com>