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 elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
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.