The ornamental design for a yarndoll, as shown and described.
FIG. 1 is a front elevational view of the yarn doll;
FIG. 2 is a rear elevational view of the yarn doll of FIG 1; and,
FIG. 3 is a perspective view of the yarn doll of FIG. 1 taken from the left side of the doll, the left and right sides of the doll being mirror images of one another.