The ornamental design for a slider for a slidefastener, as shown and described.
FIG. 1 is a perspective view of the front, bottom and right side of a slider for a slide fastener, showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof, the left side view being a mirror image thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines shown are included for the purpose of illustrating the unclaimed portions of a slider for a slide fastener and form no part of the claimed design.