The ornamental design for a lock, as shown and described.
1. Lock
1.1: Perspective [top, front and left-side]
1.2: Perspective [back, bottom and left-side]
1.3: Left
1.4: Right
1.5: Top
1.6: Bottom
1.7: Front
1.8: Back
1.9: Image of lock installed in a storage box, for Illustration
The parts of the lock colored in white and delineated in dashed line form no part of the claimed design; the storage box shown in dash-dot-dash lines in fig. 1.9is not part of the claimed design.