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

[Henry Lin]

Changed the check to require `linear` to be nonzero to prevent division by zero errors report by `ubsan`. `linear > 0` implies that `count > 0` so the count variable is no longer necessary.

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Commit messages:
 - 8333728: fix ubsan:shenandoahFreeSet.cpp runtime division by zero

Changes: https://git.openjdk.org/jdk/pull/20163/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20163&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/jdk/pull/20163.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20163/head:pull/20163

PR: https://git.openjdk.org/jdk/pull/20163