What is claimed is the ornamental design for a helmet with mouth guard, as shown and described.
FIG. 1 is a posterior view of a helmet with mouth guard implementing the new design.
FIG. 2 is a top view of the helmet with mouth guard implementing the new design; and,
FIG. 3 is a side view of the helmet with mouth guard implementing the new design.
The heavier and longer evenly spaced broken lines showing portions of the helmet with mouth guard which define the bounds of the claimed design form no part thereof. The shorter and lighter broken lines seen on the interior space of the helmet are for the purpose of illustrating environment and form no part of the claimed design.