The ornamental design for a fastener, as shown and described.
FIG. 1 is a perspective view of the fastener from above;
FIG. 2 is a perspective view of the fastener of FIG. 1 from below;
FIG. 3 is a bottom plan view of the fastener of FIG. 1;
FIG. 4 is a top plan view of the fastener of FIG. 1;
FIG. 5 is a front 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 right side elevational view of the fastener of FIG. 1; and,
FIG. 8 is a rear elevational view of the fastener of FIG. 1.
The broken lines in the drawings illustrate portions of the fastener and form no part of the claimed design. The fastener is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.