The ornamental design for a lock, as shown and described.
FIG. 1 is a front perspective view of a lock.
FIG. 2 is a front view of the lock.
FIG. 3 is a rear view of the lock.
FIG. 4 is a right-side view of the lock.
FIG. 5 is a left-side view of the lock.
FIG. 6 is a top view of the lock; and,
FIG. 7 is a bottom view of the lock.
In the figures, the solid lines show the design. The broken lines show portions of the article for illustrative purposes only. The broken lines form no part of the claimed design.