The ornamental design for a helmet, as shown and described.
FIG. 1 is a front, top and right side perspective view of a helmet showing my new design;
FIG. 2 is a rear, top and left side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a left side view thereof.
The bottom plan view of the helmet is omitted since it is unornamented and forms no part of the claimed design. The broken lines illustrate portions of the helmet and form no part of the claimed design.