The ornamental design of the fastener for strap, as shown and described.
FIG. 1 is a perspective view of a fastener for strap showing my new design;
FIG. 2 is a perspective view of the opposing side of FIG. 1;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom thereof.
The broken lines in the drawings illustrate portions of the fastener for strap and form no part of the claimed design. The broken lines showing of the strap represents the environment of the claimed design and forms no part thereof.