Disk plugin: return immediately if usage fails

closes #1297

CHANGELOG.md

@@ -44,6 +44,7 @@ time before a new metric is included by the plugin.
 - [#1258](https://github.com/influxdata/telegraf/pull/1258): Fix potential kernel plugin integer parse error.
 - [#1268](https://github.com/influxdata/telegraf/pull/1268): Fix potential influxdb input type assertion panic.
 - [#1283](https://github.com/influxdata/telegraf/pull/1283): Still send processes metrics if a process exited during metric collection.
+- [#1297](https://github.com/influxdata/telegraf/issues/1297): disk plugin panic when usage grab fails.
 
 ## v0.13.1 [2016-05-24]
 

plugins/inputs/system/ps.go

@@ -84,10 +84,10 @@ func (s *systemPS) DiskUsage(
 		mountpoint := os.Getenv("HOST_MOUNT_PREFIX") + p.Mountpoint
 		if _, err := os.Stat(mountpoint); err == nil {
 			du, err := disk.Usage(mountpoint)
-			du.Path = p.Mountpoint
 			if err != nil {
 				return nil, err
 			}
+			du.Path = p.Mountpoint
 			// If the mount point is a member of the exclude set,
 			// don't gather info on it.
 			_, ok := fstypeExcludeSet[p.Fstype]