RFR: 8259623: JfrTypeSet::_subsystem_callback is left dangling after use

[Aleksey Shipilev]

SonarCloud instance reports the bug in cases like:
  "Address of stack memory associated with local variable 'callback' is still referred to by the global variable '_subsystem_callback' upon returning to the caller. This will be a dangling reference"

For example:

static void clear_packages() {
  ClearArtifact<PkgPtr> clear;
  ClearPackageCallback callback(&clear);
  _subsystem_callback = &callback;
  do_packages();
}

I understand that `_subsystem_callback` is assigned to be used in do_packages(), but it would indeed be left dangling. The patch moves callback installation into the callback superclass itself. This way, we also guarantee that `_subsystem_callback` is not set before installation (i.e. check for overlapping callback installations), and make sure it is `NULL` after use.

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Commit messages:
 - 8259623: JfrTypeSet::_subsystem_callback is left dangling after use

Changes: https://git.openjdk.java.net/jdk/pull/3192/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3192&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259623
  Stats: 33 lines in 2 files changed: 8 ins; 12 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3192.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3192/head:pull/3192

PR: https://git.openjdk.java.net/jdk/pull/3192