RFR: 8324173: GenShen: Fix error that could cause young gcs to fail when old marking is running

[William Kemper]

Clean backport, simple fix.

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Commit messages:
 - Backport 1f7fd7cc8794c31d03e73d8adbafa9a434138250

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/17/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=17&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324173
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/17.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/17/head:pull/17

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/17