[Nestmates] RFR: 8197393: [Nestmates] Add missing NestHost attribute length check in classFileParser

[David Holmes]

webrev: http://cr.openjdk.java.net/~dholmes/8197393/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8197393

Just for the record.

The attribute-length check of the NestHost attribute was missing. Also 
added a test.

Thanks,
David