exit if the key wasn't found in the parameters list

Bug: 10761
Change-Id: I072cd3a68d852cb8dc57b9b9f807f792caba7259
Reviewed-on: https://code.wireshark.org/review/5750
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>