md: md_unregister_thread should cope with being passed NULL

Mostly md_unregister_thread is only called when we know that the
thread is NULL, but sometimes we need to check first.  It is safer
to put the check inside md_unregister_thread itself.

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 117ea5f..f30f09c 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -5382,6 +5382,8 @@
 
 void md_unregister_thread(mdk_thread_t *thread)
 {
+	if (!thread)
+		return;
 	dprintk("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
 
 	kthread_stop(thread->tsk);