We claim the ornamental design for a protective sleeve for an electronic device, as shown and described.
FIG. 1 is a perspective view of a protective sleeve for an electronic device showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines depict portions of the protective sleeve for an electronic device in which the design is embodied that form no part of the claimed design. The rear view of the protective sleeve for an electronic device is hereby disclaimed and forms no part of the claimed design.