We claim the ornamental design for a refrigerator, as shown and described.
FIG. 1 is a front perspective view of a refrigerator showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a front perspective view of the refrigerator as shown in an alternate configuration.
The broken lines in the figures depict environmental subject matter and form no part of the claimed design. The dot-dash broken lines in the figures represent an unclaimed boundary line and form no part of the claimed design.