networkd: fix segfault when cancelling callbacks

This only happens when something has gone wrong, so is not easy to hit. However,
if a bridge (say) is configured on a system without bridge support we will hit
this.

Fixes issue #299.

src/network/networkd-netdev.c

@@ -92,7 +92,7 @@ static void netdev_cancel_callbacks(NetDev *netdev) {
                         assert(netdev->manager);
                         assert(netdev->manager->rtnl);
 
-                        callback->callback(netdev->manager->rtnl, m, link);
+                        callback->callback(netdev->manager->rtnl, m, callback->link);
                 }
 
                 LIST_REMOVE(callbacks, netdev->callbacks, callback);