The ornamental design for a license-plate fastener, as shown and described.
FIG. 1 is a front perspective view of a license-plate fastener, showing my new design;
FIG. 2 is another front perspective view thereof;
FIG. 3 is a rear perspective view thereof;
FIG. 4 is a front perspective view of the centerpiece element of the license-plate fastener, shown on an enlarged scale and shown separately for ease of illustration; and,
FIG. 5 is an exploded view of one the end pieces of the license-plate fastener, shown on an enlarged scale and shown separately for ease of illustration.