We claim the ornamental design for a fan, as shown and described.
FIG. 1 is a perspective view of a fan, according to the present invention;
FIG. 2 is a right side view of the fan;
FIG. 3 is a back view of the fan;
FIG. 4 is a top view of the fan;
FIG. 5 is a front view of the fan;
FIG. 6 is a bottom view of the fan; and,
FIG. 7 is a left view of the fan.
The broken line showing of the fan blades and housing is included for the purpose of illustrating environmental structure and forms no part of the claimed design.