RFR: Handle null critical native array

[Zhengyu Gu]

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/


Test:

   hotspot_gc_shenandoah on Linux 64 (fastdebug and release)

   critical native tests on Windows (fastdebug and release)


Thanks,

-Zhengyu