The ornamental design for a valve, as shown and described.
FIG. 1 is a first perspective view of a valve showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a bottom side view thereof, the opposite side being a mirror image of the side shown;
FIG. 5 is a first side view thereof; and,
FIG. 6 is a second side view thereof.
The broken-line disclosure in the view represents a portion of the valve in which the claimed design is embodied, but which forms no part of the claimed design.