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 view of the ladder shown in FIG. 1;
FIG. 3 is a rear view of the ladder shown in FIG. 1;
FIG. 4 is a is left side view of the ladder shown in FIG. 1;
FIG. 5 is a right side view of the ladder shown in FIG. 1; and,
FIG. 6 is a top view of the ladder shown in FIG. 1.
The dashed-dot-dot-dashed broken lines in the drawings are shown for the purpose of illustrating portions of the ladder that form no part of the claimed design.