The ornamental design for a planter, as shown and described.
FIG. 1 is a perspective view of a planter showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a back elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing of various portions or other elements in the Figures are for the purpose of illustrating unclaimed portions and form no part of the claimed design.