internal/task: fix nil panic in (*internal/task.Stack).Pop

While adding some code to clear the Next field when popping from a task stack for safety reasons, the clear was placed outside of a nil pointer check.
As a result, (*internal/task.Stack).Pop panicked when the Stack is empty.

src/internal/task/queue.go

@@ -68,8 +68,8 @@ func (s *Stack) Pop() *Task {
 	t := s.top
 	if t != nil {
 		s.top = t.Next
-	}
 		t.Next = nil
+	}
 	return t
 }