The ornamental design for a dispenser for wastebags, as shown and described.
FIG. 1 is a front elevation view of the dispenser, showing my new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a side elevation view thereof;
FIG. 4 is a side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is an upper rear perspective view thereof; and,
FIG. 8 is a lower front perspective view thereof.
The dash dot lines immediately adjacent to the claimed areas in FIGS. 1 and 8 represent the boundaries of the claimed design and form no part of the claimed design.