<i18n dev> RFR: 8340554: Improve MessageFormat readObject checks

[Justin Lu]

Please review this PR which improves the readObject logic for _j.text.MessageFormat_.

No offset should be larger than the pattern length. We already ensure the offsets when consumed backwards are equal/descending. The existing first/initial check was off by 1 since it was checking against the pattern length + 1; (see L2040 and L2043).

Please see the JBS issue for further info and other test details.

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Commit messages:
 - test corrections
 - add 'good' test cases
 - init

Changes: https://git.openjdk.org/jdk/pull/21570/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21570&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/jdk/pull/21570.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21570/head:pull/21570

PR: https://git.openjdk.org/jdk/pull/21570