RFR: JDK-8304546: CompileTask::_directive leaked if CompileBroker::invoke_compiler_on_method not called

[Justin King]

Ensure `CompileTask::_directive` is not leaked when `CompileBroker::invoke_compiler_on_method` is not called. This can happen for stale tasks or when compilation is disabled.

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

Commit messages:
 - Ensure CompileTask::_directive is not leaked

Changes: https://git.openjdk.org/jdk/pull/13108/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13108&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304546
  Stats: 6 lines in 3 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13108.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13108/head:pull/13108

PR: https://git.openjdk.org/jdk/pull/13108