Do not break when given /info JSON from AUFS Docker.

Seems like the purpose of `populateInfo` is to add new data / transforms existing data into `info *DockerInfo`, right?
If it cannot find existing keys to manipulate, I think it's better to leave them alone.

This makes nagios doesn't go critical on non-meaningful errors. See [line 308](https://github.com/didip/check_docker/blob/master/check_docker.go#L308).