We claim the ornamental design for a fastener, as shown and described.
FIG. 1 is a front perspective view of the new design for a fastener;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a left view thereof; and,
FIG. 7 is a right view thereof.
The stippling is used to indicate the surface of the material from which the fastening device is made, and is not intended to indicate surface decoration.