IKEv1: send an empty KE during the IKE exchange breaking the DH calculation

Interop #1:
- initiator sends empty KE, should get invalid KE response

Interop #2:
- initiator sends valid KE
- responder sends back empty KE, which is dropped

Note: both the initiator and responder have their retransmits
supressed - in IKEv1 the responder is free to retransmit without
prompting.

regardless should not dump core
