fix panic in monitor

If you do a `while true; do docker run ubuntu ls; done` let it run for a while and `<ctrl-c>` the daemon.

When you restart the daemon you'll get a panic.


We already fixed a bug like this in https://github.com/vieux/docker/blob/1dfc44073399aadb226c1b4c1909fb15c033d44a/runtime/container.go#L811