The ornamental design for a slipcover, as shown and described.
FIG. 1 is a front perspective view of a slip cover showing my new design;
FIG. 2 is a top view thereof ;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof,
FIG. 5 is a rear view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken line showing of environmental structures, namely the couch and couch cushions in FIG. 1, is for illustrative purposes only and forms no part of the claimed design.