Add a call to starpu_task_destroy() in order to avoid a memory leak.

tests/core/starpu_worker_exists.c

@@ -82,6 +82,7 @@ main(void)
 	if (ret)
 		return EXIT_FAILURE;
 
+	starpu_task_destroy(task);
 	starpu_shutdown();
 
 	return EXIT_SUCCESS;