The ornamental design for a pump for a container, as shown and described.
FIG. 1 is a perspective view of a pump for a container showing the new design;
FIG. 2 is a front view of the new design;
FIG. 3 is a back view of the new design;
FIG. 4 is a left view of the new design;
FIG. 5 is a right view of the new design;
FIG. 6 is a top view of the new design; and,
FIG. 7 is a bottom view of the new design.
The broken-line disclosure is understood to represent portions of the article in which the claimed design is embodied, but which form no part of the claimed design.