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 cross-sectional view thereof, taken along line 8-8 in FIG. 2.
The even-dash broken lines depict portions of the Refrigerator that form no part of the claimed design. The dot-dash broken lines represent boundaries of the claimed design and form no part thereof.