The ornamental design for a bottlestopper, as shown.
FIG. 1 is a perspective view of a bottle stopper showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof. The broken lines in FIG. 7 are included for the purpose of illustrating a portion of the bottom that forms no part of the claimed design.