The ornamental design for a glowstick, as shown and described.
FIG. 1 is a perspective view of the glow stick according to the present design;
FIG. 2 is a top plan view of the design of FIG. 1;
FIG. 3 is a right side elevation view of the design of FIG. 1 with an actuator button, ribbing detail, and wrist strap attachment flange shown in broken lines, the left side being a mirror image thereof but without the wrist strap attachment flange;
FIG. 4 is a bottom plan view of the design of FIG. 1;
FIG. 5 is a front elevation view of the design of FIG. 1; and,
FIG. 6 is a rear elevation view of the design of FIG. 1.
The broken lines shown in the drawings are included for the purpose of illustrating environmental structures and form no part of the claimed design.