[m-rev.] for post-commit review: fix system RNG hanldes in the C# and Java grades

[Peter Wang]

On Fri, 19 Feb 2021 23:19:19 +1100 Julien Fischer <jfischer at opturion.com> wrote:
> 
> Fix system RNG handles in the C# and Java grades.
> 
> Ensure it is not possible to use a system RNG handle after it has been closed
> in the C# and Java grades. This brings the implementation for these backends
> into line with the /dev/urandom implementation on C.  (The other C implementation
> will be made to conform in a separate change.)
> 
> library/random.system_rng.m:
>      Add wrapper classes around the C# and Java system RNGs that allow
>      us to track if they have already been closed.

That looks fine.

Peter