The ornamental design for a handlebar, as shown and described.
FIG. 1 is a perspective view of a handlebar, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left-side elevational view thereof;
FIG. 5 is a right-side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines showing a motorcycle in FIGS. 4 and 5, and showing motorcycle controls in FIGS. 5 and 6, are included for the purpose of illustrating the handlebar in use on a vehicle, and form no part of the claimed design.