The ornamental design for a handlebar, as shown and described.
FIG. 1 is a front, left perspective view showing a handlebar according to our design;
FIG. 2 is a rear, bottom, right perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
Throughout the Figures, where shown: (i) the dotted lines showing additional structure of the handlebar are for illustrative purposes only and form no part of the claimed design; and (ii) the unshaded surfaces do not form part of the claimed design.