The ornamental design for a slipper, as shown and described.
FIG. 1 is a front perspective view of a slipper showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The short dotted lines illustrate stitching and form part of the claimed design.
As to the bottom sole (as shown in FIG. 7) only the two-dimensional aspects of the sole surface are claimed. The irregular-curved lines within the treads in FIG. 7 reflect contrasting surface texture.