RFR: 8368698: runtime/cds/appcds/aotCache/OldClassSupport.java assert(can_add()) failed: Cannot add TrainingData objects

[Igor Veresov]

That's a bit of a bug trail from [JDK-8366948](https://bugs.openjdk.org/browse/JDK-8366948). We need to check if the TD snapshot has happened before attempting to modify the data.

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Commit messages:
 - Check for snapshot

Changes: https://git.openjdk.org/jdk/pull/27593/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27593&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368698
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27593.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27593/head:pull/27593

PR: https://git.openjdk.org/jdk/pull/27593