The ornamental design for a hairstraightener, as shown and described.
FIG. 1 is a perspective view of a hair straightener showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is an enlarged view of portion 9 shown in FIG. 1.
The broken lines in the drawings depict portions of the hair straightener that form no part of the claimed design. The dash-dot broken lines in FIGS. 1 and 9 are to show an enlargement and form no part of the claimed design.