ikev2: prevent crash after no IP address

Type: fix
Ticket: VPP-1900

This fixes a crash when initiating IKE connection using interface
without any IP address.
It also ensures that the IKE connection is automatically retried once the
interface obtains an address.

Signed-off-by: jan_cavojsky <Jan.Cavojsky@pantheon.tech>
Signed-off-by: Filip Tehlar <ftehlar@cisco.com>
Change-Id: Ia1919c349e64b3a0a4198365e075e177e3ba3de5

Author: Filip Tehlar