8164585: JarFile::isMultiRelease does not return true in all cases where it should return true

[Steve Drach]

Hi,

Please review this simple change to JarFile

issue: https://bugs.openjdk.java.net/browse/JDK-8164585 <https://bugs.openjdk.java.net/browse/JDK-8164585>
webrev: http://cr.openjdk.java.net/~sdrach/8164585/webrev.00/ <http://cr.openjdk.java.net/~sdrach/8164585/webrev.00/>

Thanks,
Steve