The ornamental design for a slow cooker, as shown and described.
FIG. 1 is a top, left, front perspective view of a slow cooker embodying our design, the broken lines indicating environmental structure not forming a part of our design;
FIG. 2 is a front elevational view thereof, the broken lines indicating environmental structure not forming a part of our design;
FIG. 3 is a right side elevational view, the broken lines indicating environmental structure not forming a part of our design;
FIG. 4 is a rear elevational view thereof, the broken lines indicating environmental structure not forming a part of our design; and,
FIG. 5 is a top plan view thereof, the broken lines indicating environmental structure not forming a part of our design.