The ornamental design for a fastener, as shown and described.
FIG. 1 is a front elevation view of a fastener showing our design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is an enlarged left side elevation view thereof;
FIG. 4 is an enlarged right side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a perspective view thereof.
The fastener is shown with a symbolic break in its length. The appearance of any portion of the fastener between the break lines forms no part of the claimed design.