The ornamental design for a helmet, as shown and described.
FIG. 1 is a top front left perspective view of my new, original and ornamental design for a helmet.
FIG. 2 is a right side view thereof.
FIG. 3 is a left side view thereof.
FIG. 4 is a front view thereof.
FIG. 5 is a back view thereof.
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The details shown in broken lines form no part of the claimed design. No claim is made to the color grey since the grey tone represents surface shading only.