The ornamental design for a lock, as shown and described.
FIG. 1 is a front perspective view of a lock showing our new design;
FIG. 2 is a front view of the lock of FIG. 1;
FIG. 3 is a rear view of the lock of FIG. 1;
FIG. 4 is a right side view of the lock of FIG. 1;
FIG. 5 is a left side view of the lock of FIG. 1;
FIG. 6 is a top plan view of the lock of FIG. 1; and,
FIG. 7 is a bottom plan view of the lock of FIG. 1.
The broken lines indicate unclaimed portions and form no part of the claimed design. The break lines indicate undetermined length of the lock and the portions between the break lines form no part of the claimed design.