The ornamental design for a hairstraightener, as shown and described.
FIG. 1 is a perspective view of an embodiment of a hair straightener in an open position.
FIG. 2 is a left side elevational view of the hair straightener of FIG. 1 in an open position.
FIG. 3 is a right side elevational view of the hair straightener of FIG. 1 in an open position.
FIG. 4 is a top plan view of the hair straightener of FIG. 1 in an open position.
FIG. 5 is a bottom plan view of the hair straightener of FIG. 1 in an open position.
FIG. 6 is a front elevational view of the hair straightener of FIG. 1 in an open position; and,
FIG. 7 is a rear elevational view of the hair straightener of FIG. 1 in an open position.
The broken lines appearing in the figures are for the purpose of illustrating the environmental structure of the article of the claimed design and form no part of the claimed design.