worker: avoid potential deadlock on NearHeapLimit

It can happen that the `NearHeapLimit` callback is called while calling
the `oninit()` function on `MessagePort` construction causing a deadlock
when the `Worker::Exit()` method is called, as the `mutex_` was already
held on the `CreateEnvMessagePort()` method. To fix it, just use the
`mutex_` to protect the `child_port_data_` variable and avoid holding it
when creating the `MessagePort`.
Also, return early from `Worker::Run()` if the worker message port
could not be created.

Fixes: https://github.com/nodejs/node/issues/38208

PR-URL: https://github.com/nodejs/node/pull/38403


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>