The ornamental design for a refrigerator, as shown and described.
FIG. 1 is the front/left-side/top perspective view of a refrigerator showing our new design;
FIG. 2 is the front-side elevational view thereof;
FIG. 3 is the rear-side elevational view thereof;
FIG. 4 is the left-side elevational view thereof;
FIG. 5 is the right-side elevational view thereof;
FIG. 6 is the top-side elevational view thereof;
FIG. 7 is the bottom-side elevational view thereof; and,
FIG. 8. is the front/left-side/top perspective view thereof showing that the door is open.
The dashed broken lines illustrating portions of the refrigerator are for unclaimed environmental purposes only and form no part of the claimed design.