src: add debug check for inspector uv_async_t

Add a check to make sure start_io_thread_async is not
accidentally re-used or used when uninitialized.
(This is a bit of an odd check imo, but it helped me figure
out a real issue and it might do so again, so… why not?)

PR-URL: https://github.com/nodejs/node/pull/25777


Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>