RFR: 8333728: ubsan: shenandoahFreeSet.cpp:1347:24: runtime error: division by zero

[William Kemper]

Clean backport.

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Commit messages:
 - Backport b5575942027281166676678e2081b024ec572644

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/116/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=116&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333728
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/116.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/116/head:pull/116

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/116