The ornamental design for a valve, as shown and described.
FIG. 1 is a top perspective view of a valve showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof; and,
FIG. 6 is a top plan view thereof.
The broken lines in the drawings are included for the purpose of illustrating portions of the valve and form no part of the claimed design. The bottom of the valve forms no part of the claimed design.