The ornamental design for a planter, as shown and described.
FIG. 1 is a perspective view of a planter showing our new design;
FIG. 2 is a front view of the planter, the rear and side views being mirror images; and,
FIG. 3 is a top view of the planter.
A bottom view is of the design is flat and unornamented. Broken lines shown in the FIGS. are for illustrative purposes and form no part of the claimed design.