RFR: 8358236: [AOT] Graal crashes when trying to use persisted MDOs

[Igor Veresov]

Forgot to null out MethodData::_failed_speculations before snapshotting. As a result it gets restored with a dangling pointer.
Testing looks clean.

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Commit messages:
 - Null out MethodData::_failed_speculations before snapshot

Changes: https://git.openjdk.org/jdk/pull/25570/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25570&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358236
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25570.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25570/head:pull/25570

PR: https://git.openjdk.org/jdk/pull/25570