RFR: Fix incorrect assert logic

[William Kemper]

Fix incorrect assert logic

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Commit messages:
 - Fix incorrect assert logic

Changes: https://git.openjdk.org/shenandoah/pull/224/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=224&range=00
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/224.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/224/head:pull/224

PR: https://git.openjdk.org/shenandoah/pull/224