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; and,
FIG. 8 is a perspective reference view thereof showing the state in which the drawer for food storage located at the lower side has been drawn out and at the same time, the cover of the drawer has been slid backward.
The broken lines are for illustrative purposes only and form no part of the claimed design.