RFR [11] JDK-8204152: SignedObject throws NullPointerException for null keys with an initialized Signature object

[Valerie Peng]

Hi

Can someone please help review this fix for addressing JCK test failure?
Changes are very trivial, just check for null and thrown 
InvalidKeyException instead of letting NPE be thrown later.

Bug: https://bugs.openjdk.java.net/browse/JDK-8204152
Webrev: http://cr.openjdk.java.net/~valeriep/8204152/webrev.00/

I have verified the fix by re-running the JCK tests.

Thanks,
Valerie