The ornamental design for a helmet, as shown and described.
FIG. 1 is a front perspective view of a helmet showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is a second side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a cross sectional view taken along the line 8-8 of FIG. 6.
The broken lines in the Figures are for the purpose of illustrating portions of the helmet that form no part of the claimed design. The shade lines in the Figures show contour and not surface ornamentation.