The mutex also needs to protect the access to the state->loop variable,
since that's owned by the child thread and will be destroyed as soon as
it processes our message.
Fixes: https://github.com/libuv/libuv/issues/2625
PR-URL: https://github.com/libuv/libuv/pull/2626
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>