RFR: 8360288: Shenandoah crash at size_given_klass in op_degenerated

[William Kemper]

Both degenerated and full GCs unload classes before reclaiming unmarked humongous objects. This may result in a null klass pointer dereference when asking humongous objects for their size.

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Commit messages:
 - STW collections cannot unload classes before reclaiming garbage

Changes: https://git.openjdk.org/jdk/pull/26256/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26256&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360288
  Stats: 80 lines in 2 files changed: 42 ins; 38 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26256.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26256/head:pull/26256

PR: https://git.openjdk.org/jdk/pull/26256