The ornamental design for a fastener, as shown and described.
FIG. 1 is an isometric view of a top, front, and left side of an ornamental design for a fastener;
FIG. 2 is another isometric view of the top, front, and left side of the fastener of FIG. 1;
FIG. 3 is a front elevational view of the fastener of FIG. 1;
FIG. 4 is a rear elevational view of the fastener of FIG. 1;
FIG. 5 is a right side elevational view of the fastener of FIG. 1;
FIG. 6 is a left side elevational view of the fastener of FIG. 1;
FIG. 7 is a top plan view of the fastener of FIG. 1; and,
FIG. 8 is a bottom plan view of the fastener of FIG. 1.
The dash-dash-dash broken lines are included for the purpose of illustrating portions that form no part of the claimed design. The dash-dot-dash lines depict boundary lines and form no part of the claimed design.