The ornamental design for a hairstraightener, as shown and described.
FIG. 1 is a perspective view of a new, original and ornamental design for a hair straightener;
FIG. 2 is a right side elevation view of FIG. 1;
FIG. 3 is a left side elevation view of FIG. 1;
FIG. 4 is a front elevation view of FIG. 1;
FIG. 5 is a rear elevation view of FIG. 1;
FIG. 6 is a top plan view of FIG. 1; and,
FIG. 7 is a bottom plan view of FIG. 1.
In the drawings, the evenly-spaced broken lines illustrate portions of the hair straightener that form no part of the claimed design. The dash-dot broken lines illustrate a boundary to the claim and form no part thereof.