RFR 8230483: Shenandoah: Shenandoah assert_correct failed; Object klass pointer should not be NULL

[Zhengyu Gu]

SAP reported shenandoah assertion failure since 2019-07-14.

I could not reproduce locally. I did find an instance of failure in our 
nightlies dated back to 2019-08-27.

I believe it is caused by JDK-8226757, which changed behavior of 
SH:is_concurrent_traversal_in_progress().

We do have the fix in shenandoah/jdk, but has yet upstreamed.

Webrev: http://cr.openjdk.java.net/~zgu/JDK-8230483/webev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release) on Linux x86_64.

Thanks,

-Zhengyu