[jdk21u-dev] RFR: 8340554: Improve MessageFormat readObject checks

[Goetz Lindenmaier]

I backport this for parity with 21.0.9-oracle

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Commit messages:
 - Backport 7af46a6b424cadfe298958d774da0f21db58ecd3

Changes: https://git.openjdk.org/jdk21u-dev/pull/1987/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1987&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340554
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1987.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1987/head:pull/1987

PR: https://git.openjdk.org/jdk21u-dev/pull/1987