The ornamental design for a litterboxliner, as shown and described.
FIG. 1 is a front-top perspective view of a litter box liner showing my new design;
FIG. 2 is a front-bottom perspective view thereof;
FIG. 3 is a rear-top perspective view thereof;
FIG. 4 is a rear-bottom perspective view thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a left elevational view thereof;
FIG. 8 is a right elevational view thereof;
FIG. 9 is a top plan view thereof; and,
FIG. 10 is a bottom plan view thereof.
The broken lines shown in the drawings represent environmental matter and form no part of the claimed design. The cross hatching shading in the figures is fragmentarily shown for convenience of illustration, it being understood that the cross hatching shading is uniformly continued over the surface on which it appears.