RFR 9: 8133022: Instant.toEpochMilli() silently overflows

[Roger Riggs]

Please review a small fix and test for Instant.toEpochMilli 
ArithmeticOverflow.

Webrev:
    http://cr.openjdk.java.net/~rriggs/webrev-overflow-8133022/

Issue:
     https://bugs.openjdk.java.net/browse/JDK-8133022

Thanks, Roger