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

[Mandy Chung]

> On May 25, 2016, at 4:49 AM, 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/

Looks good.

Mandy