We claim the ornamental design for a fan, as shown and described.
FIG. 1 is a top perspective view of a fan;
FIG. 2 is a right side view thereof, the left side view thereof being identical to the left side view shown;
FIG. 3 is a front view thereof; and,
FIG. 4 is a top view thereof.
The dash-dash broken lines in the drawings are environmental lines which do not form part of the claimed design. The short dash-long dash lines are boundary lines which do not form part of the claimed design.