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 left side view thereof, the opposite side being a mirror image of the side shown;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a rear view thereof.
The broken lines showing of holes on the back side of the valve are included for the purpose of illustrating a portion of the valve and form no part of the claimed design.