RFR: 8297877: jfr functions should check JavaValue result before use

[Calvin Cheung]

Please review this changeset including:
1) change `JfrJavaSupport::get_field_global_ref()` to return a `bool` so that in `install_chunk_monitor()` in `jfrChunkRotation.cpp`, it checks the return status before doing `result.get_jobject()`.
2) add an `assert` in `JfrDCmd::print_help()` to make sure the `result` is of `T_OBJECT` type.

Passed tiers 1 - 4 testing.

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Commit messages:
 - 8297877: jfr functions should check JavaValue result before use

Changes: https://git.openjdk.org/jdk/pull/11934/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11934&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297877
  Stats: 10 lines in 4 files changed: 5 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11934.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11934/head:pull/11934

PR: https://git.openjdk.org/jdk/pull/11934