RFR: 8207775: Better management of CipherCore buffers

[Seán Coffey]

https://bugs.openjdk.java.net/browse/JDK-8207775
Simple enough fix to null out some internal buffers once they're no 
longer required.

webrev : http://cr.openjdk.java.net/~coffeys/webrev.8207775/webrev/

regards,
Sean.