The ornamental design for a valve, as shown and described.
1.-3. Valve
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Top
1.5 : Bottom
1.6 : Right
1.7 : Left
2.1 : Perspective
2.2 : Front
2.3 : Back
2.4 : Top
2.5 : Bottom
2.6 : Right
2.7 : Left
3.1 : Perspective
3.2 : Front
3.3 : Back
3.4 : Top
3.5 : Bottom
3.6 : Right
3.7 : Left
Designs 1-3: valve; the thin lines in the drawings illustrate the shapes of the surfaces; the broken lines in the drawings illustrate environmental structures that form no part of the claimed designs.