src: reset `StopTracingAgent()` before platform teardown

This makes sure that `StopTracingAgent()` is always called
before tearing down the `tracing::Agent`,
since previously its destructor might have tried to access the
agent, which would be destroyed by the (earlier) `Dispose()` call.

PR-URL: https://github.com/nodejs/node/pull/25472


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>