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 front view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a right side view thereof, the left side view being a mirror image thereof;
FIG. 6 is an enlarged view thereof, taken from FIG. 1; and,
FIG. 7 is an enlarged view thereof taken from FIG. 2.
The broken lines illustrate portions of the helmet that form no part of the claimed design. The dot-dot dash broken lines of FIGS. 1, 2, 6, and 7 are included to show partially enlarged views only and form no part of the claimed design.