I claim the ornamental design for a ladder, as shown and described.
FIG. 1 is a perspective view of a ladder, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is an in-use image of the ladder on a van.
The broken lines shown in the figures are for the purpose of illustrating a portion of the ladder that forms no part of the claimed design. The broken lines showing a portion of a van in FIG. 8 are for the purpose of illustrating environment and form no part of the claimed design.