The ornamental design for a pump casing, as shown and described.
FIG. 1 is a front and upper perspective view of the design.
FIG. 2 is a left side and upper perspective view of the design.
FIG. 3 is a front view of the design.
FIG. 4 is a rear view of the design.
FIG. 5 is a left side view of the design.
FIG. 6 is a right side view of the design.
FIG. 7 is a top plan view of the design; and,
FIG. 8 is a bottom plan view of the design.
The broken lines in FIGS. 1, 2 and 7 are included for the purpose of illustrating a portion of the pump casing that forms no part of the claimed design.