The ornamental design for a slipcover, as shown and described.
FIG. 1 is a perspective view of a slip cover showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof, the left side being a mirror image;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken line showing of environmental structures, namely the couch and couch cushion in FIG. 1, is for illustrative purposes only and forms no part of the claimed design.