Avoid NPE in "JClouds slave cleanup thread"

```
INFO: Terminating slave-name slave
Exception in thread "JClouds slave cleanup thread" java.lang.NullPointerException
        at jenkins.plugins.jclouds.compute.JCloudsComputer.deleteSlave(JCloudsComputer.java:62)
        at jenkins.plugins.jclouds.compute.JCloudsCleanupThread.execute(JCloudsCleanupThread.java:69)
        at hudson.model.AsyncPeriodicWork$1.run(AsyncPeriodicWork.java:54)
        at java.lang.Thread.run(Thread.java:745)
```