src: plug trace file file descriptor leak

Close existing file descriptors before overriding
the field with new ones.

Also, use `nullptr` as the loop for these synchronous
operations, since they do not run on the same thread
as the `uv_run()` call for the previously used loop.

PR-URL: https://github.com/nodejs/node/pull/21867


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>