[jdk21u-dev] RFR: 8334867: Add back assertion from JDK-8325494

[Martin Doerr]

This PR adds the assertion which has been omitted in https://github.com/openjdk/jdk21u-dev/pull/734. See the discussion there and in the JBS issue for details.

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Commit messages:
 - 8334867: Add back assertion from JDK-8325494

Changes: https://git.openjdk.org/jdk21u-dev/pull/789/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=789&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334867
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/789.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/789/head:pull/789

PR: https://git.openjdk.org/jdk21u-dev/pull/789