The ornamental design for a planter, as shown and described.
FIG. 1 is a perspective view of the planter of the present invention;
FIG. 2 is a right side elevation view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a rear elevation view thereof; and,
FIG. 6 is a second top plan view thereof shown.
The evenly-spaced broken lines depict environmental subject matter only and the unevenly-spaced broken lines depict the boundaries of the claim. Neither type of broken line forms any part of the claimed design.