RFR: 8265759: Shenandoah: Avoid race for referent in assert

[Zhengyu Gu]

On Thu, 22 Apr 2021 15:17:25 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> There is an assert is ShenandoahReferenceProcessor::drop() which is racy: when referent is concurrently cleaned it may enter mark-bit-map with NULL even though we just checked that the referent is not NULL.
> 
> Testing:
>  - [x] hotspot_gc_shenandoah

Marked as reviewed by zgu (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/3632