The ornamental design for a sandbag, as shown and described.
FIG. 1 is a front perspective view of a sandbag, showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is an alternate perspective view thereof; shown in a state of use.
The broken lines showing a cylinder pole in FIG. 9 depict environmental subject matter that forms no part of the claimed design. The remaining broken lines depict stitching that form no part of the claimed design.