This PR fixes an issue introduced in Nomad 0.6.0 due to
https://github.com/shirou/gopsutil/issues/420. The issue arised from the
fact that the Windows stats from gopsutil reports CPUs in
percentages where we expected ticks.

This commit protects against finding `0.NaN` tokens in JSON streams
because of infinity representation on serialization.

To reproduce logspam:

```
$ docker plugin install --grant-all-permissions vieux/sshfs
$ nomad agent -dev
...
2017/08/25 17:09:03.282868 [WARN] client: error fetching host disk usage stats for /var/lib/docker/plugins/a8b4a69b07e5180f828d19e1e9e102ccc0e26f9c9939eaef85357260c30b20a7/rootfs/mnt/volumes: permission denied
... repeats every collection period ...
```