JDK 8 RFR 8016252: More defensive HashSet.readObject

[Brian Burkhalter]

Please review and comment at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8016252
Webrev:	http://cr.openjdk.java.net/~bpb/8016252/

Summary
* Improve validation checks in HashSet.readObject().

Would this change imply updating the serialVersionUID?

Thanks,

Brian