The ornamental design for a slipper, as shown and described.
FIG. 1 is a front elevation view of a slipper;
FIG. 2 is a rear elevation view of the slipper of FIG. 1;
FIG. 3 is a left side elevation view of the slipper of FIG. 1;
FIG. 4 is a right side elevation view of the slipper of FIG. 1;
FIG. 5 is a top plan view of the slipper of FIG. 1; and,
FIG. 6 is a bottom plan view of the slipper of FIG. 1.
The broken lines as shown in FIGS. 1-4 are for the purpose of illustrating a tread pattern on the soles of the slipper and forms no part of the claimed design.