The ornamental design for a body harness, as shown and described.
FIG. 1 is a rear perspective view of a body harness showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a left side elevation view, with the right side elevation being a mirror image thereof;
FIG. 4 is rear elevation view thereof.
The broken lines in the drawings represent environmental subject matter and form no part of the claimed design. Any portions of the harness that are not shown in the drawings are not part of the claimed design.