sched: initialise kernel_thread parent ptr

sched/task.c

@@ -39,6 +39,7 @@ kern_status_t setup_kernel_task(void)
 	kernel_thread->tr_id = 0;
 	kernel_thread->tr_prio = PRIO_NORMAL;
 	kernel_thread->tr_state = THREAD_READY;
+	kernel_thread->tr_parent = __kernel_task;
 
 	unsigned long flags;
 	task_lock_irqsave(__kernel_task, &flags);
@@ -80,5 +81,6 @@ task_t *task_from_pid(unsigned int pid)
 
 task_t *current_task(void)
 {
-	return current_thread()->tr_parent;
+	thread_t *thr = current_thread();
+	return thr ? thr->tr_parent : NULL;
 }

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;