The ornamental design for a lock, as shown and described.
1. Lock
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
Fig. 1.1 is a perspective view of the design; fig. 1.2 is a front view thereof; fig. 1.3 is a back view thereof; fig. 1.4 is a left side view thereof; fig. 1.5 is a right side view thereof; fig. 1.6 is a top view thereof; and fig. 1.7 is a bottom view thereof.