8228338: tools/pack200/TimeStamp.java fails with NullPointerException

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8228338
http://cr.openjdk.java.net/~bpb/8228338/webrev.00/

Another slip up due to not having tested the JDK-8067801 patch sufficiently.

Thanks,

Brian