RFR: 8284734: Undiscovered references if using ReferentBasedDiscovery

Tue Apr 12 08:27:07 UTC 2022

Small change of fixing a crash if using `ReferentBasedDiscovery`.

Test: hotspot_gc

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Commit messages:
 - improve comment
 - remove tracing log
 - ref-policy

Changes: https://git.openjdk.java.net/jdk/pull/8196/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8196&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284734
  Stats: 21 lines in 1 file changed: 2 ins; 12 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8196.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8196/head:pull/8196

PR: https://git.openjdk.java.net/jdk/pull/8196