The ornamental design for a refrigerator, as shown and described.
FIG. 1 is a 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 plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a fragmented enlarged perspective view of the upper portion of the refrigerator; and,
FIG. 9 is a fragmented enlarged perspective view of the lower portion of the refrigerator.
The broken lines showing the remainder of the refrigerator are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.