The ornamental design for a reflector for light assembly, as shown and described.
FIG. 1 is a front elevation view of the reflector for a light assembly; and,
FIG. 2 is a front elevation view of the reflector for a light assembly showing the environment thereof.
The broken lines are shown for illustrative purposes only and form no part of the claimed design. The claim is solely directed to the front view of the reflector for a light assembly.