I claim the ornamental design for a lock, as shown and described.
FIG. 1 is a top right front perspective view of a lock showing an embodiment of my new design, the top left rear view perspective view being a mirror image thereof.
FIG. 2 is a left side elevational view thereof.
FIG. 3 is a right side elevational view thereof.
FIG. 4 is a front elevational view thereof.
FIG. 5 is a rear elevational view thereof; and,
FIG. 6 is a top plan view thereof, the bottom plan view being a mirror image thereof.