RFR 8243592: Subject$SecureSet::contains(null) is suboptimal

[Weijun Wang]

Please take a review at

   http://cr.openjdk.java.net/~weijun/8243592/webrev.00/

This is helpful if we do any set arithmetic between 2 Subject objects.

No new regression test, I intend to add a noreg-trivial label.

*Tigran*: Please confirm you are OK with the "Contributed-by" line.

Thanks,
Max