I claim the ornamental design for a bag stand, as shown and described.
FIG. 1 is a top front perspective view of a bag stand showing my new design.
FIG. 2 is a top plan view thereof, taken along the line II-II from FIG. 1.
FIG. 3 is a bottom plan view thereof, taken along the line III-III from FIG. 1.
FIG. 4 is a front side elevational view thereof. The back side elevational, right side elevational and left side elevational views being identical to the front side elevational view.
FIG. 5 is an exploded perspective view of the bag stand; and,
FIG. 6 is an exploded perspective view of the bag stand showing a bag mounted therein in dotted line. The dotted line bag is shown for environmental structure only and forms no part of the claimed design.