The ornamental design for a fastener, as shown and described.
FIG. 1 is a front elevation view of a fastener.
FIG. 2 is a back elevation view of the fastener thereof.
FIG. 3 is a side elevation view of the fastener thereof.
FIG. 4 is an opposed side elevation view of the fastener thereof.
FIG. 5 is a top plan view of the fastener thereof; and,
FIG. 6 is a bottom plan view of the fastener thereof.
The broken line circles of FIGS. 1 and 2 and the broken line threading are included for the purpose of showing portions of the article and forms no part of the claimed design.