The ornamental design for a valve, as shown and described.
FIG. 1 is a perspective view of a valve showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a right side view thereof, the opposite side being a mirror image of the side shown; and,
FIG. 6 is a rear view thereof.
The broken line showing a hole on the rear side of the valve is included for the purpose of illustrating a portion thereof and forms no part of the claimed design.