IKEv2: send zero KE during the IKE exchange breaking the DH calculation

Interop #1:
- initiator sends zero KE
- responder returns zero KE

Interop #2:
- initiator sends valid KE
- responder sends zero KE

In both cases, since the initiator performs DH before the responder,
it is the initiator that will detect that DH failed and logged.

More importantly, it should not dump core.
