I claim the ornamental design for a heating pad, as shown and described.
FIG. 1 is a perspective view of a heating pad, showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is another perspective view thereof showing a configuration of use.
The broken lines on the top and bottom surfaces of the heating pad shown in FIGS. 1, 6, and 7 depict stitching. The broken lines shown in FIG. 8 depict unclaimed environment that do not form a part of the claimed design.