The ornamental design for a helmet, as shown and described.
FIG. 1 is a top front perspective view of a helmet;
FIG. 2 is a right elevation view of FIG. 1, the left elevation view being a mirror image thereof;
FIG. 3 is a front side elevation view of FIG. 1;
FIG. 4 is a rear side elevation view of FIG. 1;
FIG. 5 is a top plan view of FIG. 1; and,
FIG. 6 is a bottom plan view of FIG. 1.
The evenly dashed broken lines shown in the drawings depict portions of the helmet that form no part of the claimed design, while the dash-dot-dash lines represent the bounds of the claimed design.