RFR: 8357976: GenShen crash in swap_card_tables: Should be clean

[William Kemper]

Clean backport. Missing this change triggers the assertion fixed in https://github.com/openjdk/shenandoah-jdk21u/pull/203.

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Commit messages:
 - Backport 382f870cd53d69dfc1d2b96150aa2c026e47f642

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/204/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=204&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357976
  Stats: 17 lines in 2 files changed: 10 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/204.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/204/head:pull/204

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/204