osx,fsevent: clear pointer when deleting it [NFCI]

Set this to NULL just before disposing it to make mistakes more
painfully obvious, hopefully.

PR-URL: https://github.com/libuv/libuv/pull/2626
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/fsevents.c

@@ -747,6 +747,8 @@ static void* uv__cf_loop_runner(void* arg) {
                          state->signal_source,
                          *pkCFRunLoopDefaultMode);
 
+  state->loop = NULL;
+
   return NULL;
 }