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

[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

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Commit messages:
 - 8265759: Shenandoah: Avoid race for referent in assert

Changes: https://git.openjdk.java.net/jdk/pull/3632/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3632&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265759
  Stats: 5 lines in 1 file changed: 3 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