RFR: Check for NULL in is-alive-closures

[Aleksey Shipilev]

On 02/02/2018 03:19 PM, Roman Kennke wrote:
> HmmmHmm. Ok:
> 
> http://cr.openjdk.java.net/~rkennke/fixisalivecl/webrev.01/

Looks good.

We shall common Shenandoah*IsAlive closures at some point.

Thanks,
-Aleksey