RFR: Handle null critical native array

[Aleksey Shipilev]

On 03/29/2018 06:58 PM, Zhengyu Gu wrote:
> Early patch did not handle NULL critical native array properly.
> 
> Even null arrays are not pinned, but we still need to track them.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/sh-critical-native-null-array/webrev.00/

Looks good.

-Aleksey