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