The ornamental design for a softbox, as shown and described.
FIG. 1 is a top, front and right side perspective view of a softbox, showing our new design;
FIG. 2 is a bottom, rear and left side perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is an enlarged view of portion 9 shown in FIG. 2.
The broken lines shown in the drawings are included for the purpose of illustrating portions of the softbox that form no part of the claimed design. In addition, the dot-dash broken lines in FIGS. 2 and 9 depict the boundaries of the enlargements that form no part of the claimed design.