The ornamental design for a helmet, as shown and described.
FIG. 1 is a front perspective view of a helmet showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left-side elevational view thereof;
FIG. 6 is a right-side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to portions of the helmet that form no part of the claimed design.