Because the `derived` flag was not reset (it's set after the initial
IKE_SA_INIT exchange), no keys would get derived when sending
IKE_INTERMEDIATE during the next try. As there is then no `aead_t`
available, encrypting the message would fail and the initiation would
remain stuck.
Fixes: 0d49ddec2e ("ike-init: Add support for multiple key exchanges")