I claim the ornamental design for a novelty button, as shown and described.
FIG. 1 is a perspective view of a novelty button, showing the 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; and,
FIG. 7 is a bottom plan view thereof.
The shading pattern on the top surface in FIGS. 1-6 represent the color green. The broken lines showing a speaker grill and battery compartment in FIG. 7 form no part of the claimed design.