message: fix gtp mode with child_N rewrite
diff: @@ -122,7 +122,7 @@ class MCTSPlayerMixin:
  self.root.children_as_pi(self.root.position.n > self.temp_threshold))
  self.qs.append(self.root.Q) # Save our resulting Q.
  self.comments.append(self.root.describe())
  - self.root = self.root.add_child(coords.flatten_coords(c))
  + self.root = self.root.maybe_add_child(coords.flatten_coords(c))
  self.position = self.root.position # for showboard
  del self.root.parent.children
  return True # GTP requires positive result.
  