RFR: 8319961: JvmtiEnvBase doesn't zero _ext_event_callbacks [v2]

[Roman Marchenko]

> Zero'ing memory of extension event callbacks

Roman Marchenko has updated the pull request incrementally with one additional commit since the last revision:

  Fixing review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16647/files
  - new: https://git.openjdk.org/jdk/pull/16647/files/aa3bfaec..2ef1e341

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16647&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16647&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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