[jdk21u] RFR: 8319961: JvmtiEnvBase doesn't zero _ext_event_callbacks

[Roman Marchenko]

Clean backport, no regressions.

-------------

Commit messages:
 - Backport 97ea5bf0ffafaf8009c19483b9a9b1c30401cf9a

Changes: https://git.openjdk.org/jdk21u/pull/370/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=370&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319961
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk21u/pull/370.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/370/head:pull/370

PR: https://git.openjdk.org/jdk21u/pull/370