src: fix TracingController cleanup

This fixes an incorrect deletion of the `TracingController` instance,
which in some environments could cause an error about an invalid
pointer passed to `free()`. The `TracingController` instance is
actually owned by a `unique_ptr` member of the platform, so calling
`platform::SetTracingController(nullptr)` is the correct way to
delete it. But before that, the `TraceBuffer` must be deleted in
order for the tracing loop to exit; that is accomplished by calling
`TracingController::Initialize(nullptr)`.

PR-URL: nodejs#10623
Reviewed-By: Matthew Loring <mattloring@google.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

src/tracing/agent.cc

@@ -56,7 +56,9 @@ void Agent::Stop() {
   // Perform final Flush on TraceBuffer. We don't want the tracing controller
   // to flush the buffer again on destruction of the V8::Platform.
   tracing_controller_->StopTracing();
-  delete tracing_controller_;
+  tracing_controller_->Initialize(nullptr);
+  tracing_controller_ = nullptr;
+
   // Thread should finish when the tracing loop is stopped.
   uv_thread_join(&thread_);
   v8::platform::SetTracingController(platform_, nullptr);