Null safety.

Observed in a test against 1.480:
… hudson.model.Executor run
SEVERE: Unexpected executor death
java.lang.NullPointerException
	at hudson.model.Computer.getNode(Computer.java:473)
	at hudson.slaves.SlaveComputer.getNode(SlaveComputer.java:177)
	at hudson.slaves.SlaveComputer.getRetentionStrategy(SlaveComputer.java:609)
	at hudson.slaves.SlaveComputer.taskCompleted(SlaveComputer.java:285)
	at hudson.model.queue.WorkUnitContext.synchronizeEnd(WorkUnitContext.java:140)
	at hudson.model.Executor.run(Executor.java:253)