[11] RFR: JDK-8197441: Signature#initSign/initVerify for an invalid private/public key fails with ClassCastException for SunPKCS11 provider

[Valerie Peng]

Anyone can help review this trivial fix? It'll just take a minute or so.
Essentially, the fix is to re-throw with InvalidKeyException when the 
casting failed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8197441
Webrev: http://cr.openjdk.java.net/~valeriep/8197441/webrev.00/

Thanks,
Valerie