The ornamental design for stopper, as shown and described.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
1. Stopper
1.1 is a side view shown in a horizontal position.
1.2 is a top perspective view.
1.3 is another side view shown in a horizontal position.