RFR: 8359472: JVM crashes when attaching a dynamic agent before JVMTI_PHASE_LIVE [v12]

[Francesco Andreuzzi]

> In this PR I add a check to prevent debug builds to crash when an agent tries to attach while the JVM is not in live phase.
> 
> Passes tier1 and tier2 (fastdebug).

Francesco Andreuzzi has updated the pull request incrementally with one additional commit since the last revision:

  nullptr

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27766/files
  - new: https://git.openjdk.org/jdk/pull/27766/files/93667b80..2e628c60

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27766&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27766&range=10-11

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27766.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27766/head:pull/27766

PR: https://git.openjdk.org/jdk/pull/27766