IOThread: use FatalError() on g_thread_create() error

New GLib versions don't fail.

test/run_output.cxx

@@ -210,11 +210,7 @@ int main(int argc, char **argv)
 	main_loop = new EventLoop(EventLoop::Default());
 
 	io_thread_init();
-	if (!io_thread_start(&error)) {
-		g_warning("%s", error->message);
-		g_error_free(error);
-		return EXIT_FAILURE;
-	}
+	io_thread_start();
 
 	/* initialize the audio output */