RFR: 8319961: JvmtiEnvBase doesn't zero _ext_event_callbacks

[Roman Marchenko]

Zero'ing memory of extension event callbacks

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Commit messages:
 - JDK-8319961: JvmtiEnvBase doesn't zero _ext_event_callbacks

Changes: https://git.openjdk.org/jdk/pull/16647/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16647&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319961
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16647.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16647/head:pull/16647

PR: https://git.openjdk.org/jdk/pull/16647