src: avoid race condition in tracing code

`json_trace_writer_` is protected by `stream_mutex_`,
but one access to it was not guarded by a lock on said mutex.

Refs: https://github.com/nodejs/node/issues/25512

PR-URL: https://github.com/nodejs/node/pull/25624


Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>