The ornamental design for a refrigerator, as shown and described.
FIG. 1 is a top 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;
FIG. 8 is an enlarged view of the encircled portion 8 of FIG. 1; and,
FIG. 9 is an enlarged bottom perspective view of the portion thereof shown in FIG. 8.
The broken lines illustrating portions of the refrigerator form no part of the claimed design. The dot-dash lines encircling the portions of FIGS. 1, 8 and 9 for the purpose of illustrating the enlarged view thereof form no part of the claimed design.