src: fix missing handlescope bug

Fix a regression that was introduced in commit 5886e204 ("inspector: track async stacks when necessary") and that I overlooked during review: the persistent handle with the callback must be rematerialized after the v8::HandleScope is created, not before.

Apparently test/sequential/test-inspector-async-call-stack.js has no test coverage for this scenario and I'm out of good ideas on how to create a concise and reliable test case.

Fixes: https://github.com/nodejs/node/issues/17496 CI: https://ci.nodejs.org/job/node-test-pull-request/11962/