[15] RFR 8238574: Shenandoah: Assertion failure due to missing null check

[Zhengyu Gu]

Please review this small change that adds a null check before calling 
keep alive barrier to avoid assertion failure.

Native barrier may return null for a not null oop, if it is dead.

Bug: https://bugs.openjdk.java.net/browse/JDK-8238574
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8238574/webrev.00/

Test:
   hotspot_gc_shenandoah, vmTestbase_nsk_jdi where the problem was observed.

Thanks,

-Zhengyu