The ornamental design for a valve, as shown and described.
1. Valve
1.1: Perspective
1.2: Front
1.3: Back
1.4: Top
1.5: Bottom
1.6: Right
1.7: Left
Design 1: a valve; the thin lines in the drawings illustrate the shape of the surface; the broken lines in the drawings illustrate environmental structures that form no part of the claimed design, more specifically the complete upper solenoid in Figure 1.1 through 1.7 does not form a part of the claimed design, including all aspects of the electrical connector shown in Figure 1.6.