The ornamental design for a bun for hotdogs, as shown and described.
FIG. 1 is a front elevational view of a bun for hotdogs showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a left side elevational view thereof; and,
FIG. 4 is a front perspective view thereof, the broken lines showing an arrangement of additional buns are for illustrative purposes only and form no part of the claimed design.