RFR: JDK-8162723 : Array index overflow in Base64 utility class

[Sean Mullan]

Please review this change to fix a potential array index overflow in the 
XML security Base64 encoding class. This is a direct import of the 
corresponding patch that has already been made to the Apache code.

webrev: http://cr.openjdk.java.net/~mullan/webrevs/8162723/webrev.00/

Thanks,
Sean