The ornamental design for a handlebar, as shown and described.
FIG. 1 is a front perspective view of a handlebar showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawings illustrate portions of the handlebar that form no part of the claimed design. Additionally, the handlebar is shown with symbolic breaks in some of its lengths. The appearance of any portion of the article between these break lines forms no part of the claimed design.