The ornamental design for a fastener, as shown and described.
FIG. 1 is a perspective view of a fastener showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawings illustrate portions of the fastener that form no part of the claimed design. The dot-dot-dash broken lines in the drawings define the bounds of the claim and form no part thereof. None of the broken lines form any part of the claimed design.