The ornamental design for a ladder, as shown and described.
FIG. 1 is a perspective view of a ladder showing our new design;
FIG. 2 is a front elevation view of the ladder shown in FIG. 1;
FIG. 3 is a right side elevation view of the ladder shown in FIG. 1, the left side elevation view of the ladder being a mirror image of the right side elevation view;
FIG. 4 is a is rear elevation view of the ladder shown in FIG. 1;
FIG. 5 is a top plan view of the ladder shown in FIG. 1; and,
FIG. 6 is a bottom plan view of the ladder shown in FIG. 1.
The broken lines shown in the figures are for the purpose of illustrating portions of the ladder and form no part of the claimed design.