The ornamental design for a fastener for a heatsink and fan assembly, as shown and described.
FIG. 1 is a perspective view of a fastener for a heat sink and fan assembly of our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view thereof being mounted to a heat sink and a fan, wherein the heat sink and the fan are shown in broken lines for illustrative environmental purposes only and form no part of the claimed design.