Check if colliding rekey actually created an IKE_INIT

In some cases (especially if a child is half-open) the colliding
rekey-job might not have created the ike_init member. If so, the
nonce check fails with SIGSEGV.

src/libcharon/sa/tasks/ike_rekey.c

@@ -241,10 +241,14 @@ static status_t process_i(private_ike_rekey_t *this, message_t *message)
 	if (this->collision &&
 		this->collision->get_type(this->collision) == IKE_REKEY)
 	{
-		chunk_t this_nonce, other_nonce;
-		host_t *host;
 		private_ike_rekey_t *other = (private_ike_rekey_t*)this->collision;
 
+		/* ike_init can be NULL, if child_sa is half-open */
+		if (other->ike_init)
+		{
+			host_t *host;
+			chunk_t this_nonce, other_nonce;
+
 			this_nonce = this->ike_init->get_lower_nonce(this->ike_init);
 			other_nonce = other->ike_init->get_lower_nonce(other->ike_init);
 
@@ -287,6 +291,7 @@ static status_t process_i(private_ike_rekey_t *this, message_t *message)
 				other->new_sa = NULL;
 				return SUCCESS;
 			}
+		}
 		/* set threads active IKE_SA after checkin */
 		charon->bus->set_sa(charon->bus, this->ike_sa);
 	}