RFR 8203368: ObjectInputStream filterCheck method throws NullPointerException

[Roger Riggs]

Please review a fix for the serial filter mechanism to prevent a NPE in 
the case where
ObjectInputStream has been subclassed and the input stream is not available.
Webrev:
  http://cr.openjdk.java.net/~rriggs/webrev-ois-override-8203368/

Thanks, Roger