The ornamental design for a fan, as shown and described.
FIG. 1 is an isometric view of the fan showing my new design;
FIG. 2 is a side elevational view of the fan of FIG. 1; and,
FIG. 3 is a top plan view of the fan of FIG. 1.
The broken line showing of portions of the fan is included for the purpose of illustrating environmental structure and forms no part of the claimed design.