guard against missing ipv6 information

breaks if the server is in a stopped state, in which case the
information is not available.

fixes scaleway#42

scaleway/resource_server.go

@@ -248,7 +248,7 @@ func resourceScalewayServerRead(d *schema.ResourceData, m interface{}) error {
 	d.Set("private_ip", server.PrivateIP)
 	d.Set("public_ip", server.PublicAddress.IP)
 
-	if server.EnableIPV6 {
+	if server.EnableIPV6 && server.IPV6 != nil {
 		d.Set("public_ipv6", server.IPV6.Address)
 	}