I claim the ornamental design for a helmet, as shown and described.
FIG. 1 is a view of the helmet in perspective at an angle from the front and left-hand side, with respect to the front, showing the new design;
FIG. 2 is a view of the helmet in perspective at an angle from the rear and right-hand side;
FIG. 3 is a view of the helmet from the left-hand side;
FIG. 4 is a view of the helmet from the right-hand side;
FIG. 5 is a view of the helmet from the front;
FIG. 6 is a view of the helmet from the rear;
FIG. 7 is a view of the helmet from the bottom; and,
FIG. 8 is a view of the helmet from the top.
The broken lines immediately adjacent to the claim depict the bounds of the claimed design, while all other broken lines are directed to environment. The broken lines form no part of the claimed design.