sched: initialise kernel_thread parent ptr

sched/thread.c

@@ -32,6 +32,10 @@ void thread_free(thread_t *thr)
 thread_t *current_thread(void)
 {
 	cpu_data_t *cpu = get_this_cpu();
+	if (!cpu) {
+		return NULL;
+	}
+
 	thread_t *out = cpu->c_current_thread;
 	put_cpu(cpu);
 	return out;