The ornamental design for a shield for a helmet, as shown and described.
FIG. 1 is a front, top, and right side perspective view of a shield for a helmet;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of portions of a shield for a helmet is included for the purpose of illustrating environmental structure and forms no part of the claimed design.