RFR: 8265759: Shenandoah: Avoid race for referent in assert [v2]

[Roman Kennke]

> 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

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Fix formatting

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3632/files
  - new: https://git.openjdk.java.net/jdk/pull/3632/files/73a36ca4..18b8a6f5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3632&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3632&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3632.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3632/head:pull/3632

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