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 an enlarged view of the portion 8 in FIG. 1.
The even-dash broken lines illustrating portions of the refrigerator form no part of the claimed design. The dot-dash broken lines encircling enlargement portions of the refrigerator form no part of the claimed design.