The ornamental design for a supportpad for a walking stick, as shown and described.
FIG. 1 is a perspective view of a support pad for a walking stick showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view of the support pad for the walking stick, wherein the broken line showing of the walking stick is illustrative of environment structure and forms no part of the claimed design.