RFR: 8374371: Failed assertion in G1HeapRegion gtest

[Kim Barrett]

Please review this simple change to the G1HeapRegion gtest to eliminate a
failed assertion when run in some configurations.

Testing: mach5 tier1. Locally (linux-aarch64) ran the test directly, in a
configuration that was failing and no longer fails after the change.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/29000/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29000&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374371
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29000.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29000/head:pull/29000

PR: https://git.openjdk.org/jdk/pull/29000