The ornamental design for a valve, as shown and described.
FIG. 1 is a top view of the valve;
FIG. 2 is bottom view of the valve of FIG. 1;
FIG. 3 is a left side view of the valve of FIG. 1;
FIG. 4 is a right-side view of the valve of FIG. 1;
FIG. 5 is a front view of the valve of FIG. 1; and,
FIG. 6 is a back view of the valve of FIG. 1.
The broken lines shown in the figures are for the purpose of illustrating environment and form no part of the claimed design. The valve is shown with symbolic breaks in its length. The appearance of any portion of the article between the break lines form no part of the claimed design.