The ornamental design for a fan, as shown and described.
FIG. 1 is a first perspective view of the fan showing our new design in a first configuration;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view of the fan showing our new design in a second configuration;
FIG. 4 is rear view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment; the broken lines form no part of the claimed design.