RFR: 8230709: Array index out of bounds in ES6 mode

[Hannes Wallnöfer]

Please review this off-by-one array index fix that was recently reported on 
this list:

JBS: https://bugs.openjdk.java.net/browse/JDK-8230709
Webrev: http://cr.openjdk.java.net/~hannesw/8230709/webrev.00/

Thanks,
Hannes