I claim the ornamental design for a headphone stand, as shown and described.
FIG. 1 is a top perspective view of a headphone stand, showing my new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a right-side elevational view thereof, the left-side elevational view being a mirror image thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is another perspective view of the headphone stand shown with headphones.
The broken line showing of headphones in FIG. 8 depicts environmental matter and forms no part of the claimed design. The broken lines showing a circular feature in FIGS. 2 and 7 depict portions of the headphone stand that form no part of the claimed design.