The ornamental design for a valvestem, as shown and described.
FIG. 1 is a perspective view of a valve stem showing my new design, said valve stem being shown mounted at a top of a pressurized gas container shown in broken outline which does not form part of the design;
FIG. 2 is a perspective side view of the valve stem;
FIG. 3 is a perspective end view of the valve stem;
FIG. 4 is an enlarged perspective view of the valve stem;
FIG. 5 is an enlarged plan view of the valve stem; and,
FIG. 6 is a perspective view of the valve stem according to the design, said valve stem being shown incorporated in an adapter for mounting at a top of a pressurized gas container shown in broken outline which does not form part of the design.