We claim the ornamental design for a case for a portable electronic device, as shown and described.
FIG. 1 is a front perspective view of a case for a portable electronic device according to the claimed design.
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 elevational view thereof.
The broken lines in the drawings are shown to illustrate the portions of a case in which the design is embodied and form no part of the claimed design.