8156142: ModuleReader instances don't always throw NPE for passed null args

[Chris Hegarty]

> On 25 May 2016, at 12:49, Alan Bateman <Alan.Bateman at oracle.com> wrote:
> 
> 
> Another trivial patch, this time it's the ModuleReader for the system modules is missing a null check so that it doesn't throw the expected NPE. The patch is to mostly just expand test coverage.
>  http://cr.openjdk.java.net/~alanb/8156142/webrev/

I think this is fine. Does the @implSpec on release(ByteBuffer) need
to be updated too?

-Chris.