The ornamental design for a refrigerator, as shown and described.
FIG. 1 is a perspective view of a refrigerator in accordance with our design.
FIG. 2 is a front view thereof.
FIG. 3 is a right side view thereof.
FIG. 4 is a left side view thereof; and,
FIG. 5 is a top view thereof.
It will be understood that the bottom and rear views are not seen in use. It will be further understood that no claim is made to the hidden line showing throughout the drawings.
The bottom and rear views of the refrigerator are not shown, and therefore not claimed; the broken lines in the drawings depict unclaimed portions of the refrigerator only and form no part of the claimed design.