The ornamental design for a helmet, as shown and described.
FIG. 1 is a top, front and left side perspective view of a helmet showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawings depict portions that are not part of the claimed design, but rather show the environment in which bthe design is associated.