The ornamental design for a fan, as shown and described.
FIG. 1 is a front view of the fan;
FIG. 2 is a rear view thereof;
FIG. 3 is a front perspective view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is another side view thereof;
FIG. 6 is another side view thereof; and,
FIG. 7 is another side view thereof.
The broken line showing of portions of a fan is included for the purpose of illustrating environmental structure and forms no part of the claimed design.