The ornamental design for a fan, as shown and described.
FIG. 1 is a first perspective view of a fan showing my new new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is an enlarged view of the selected portion in FIG. 1.
The dash-dash broken lines shown in the drawings depict portions of the fan in which the design is embodied that form no part of the claimed design. The dot-dash broken lines surrounding the portion of the fan in FIG. 1 and FIG. 9 form no part of the claimed design.