The ornamental design for a headbandpillow, as shown and described.
FIG. 1 is a front view of a headband pillow of the claimed design;
FIG. 2 is a right side view of FIG. 1;
FIG. 3 is a right perspective back view of FIG. 1;
FIG. 4 is a back view of FIG. 1;
FIG. 5 is a left side view of FIG. 1; and,
FIG. 6 is a top view of FIG. 1.
The broken line outline of the partial human figure assists in illustrating how the headband pillow may appear when in a position of use, but forms no part of the claimed design.