The ornamental design for a pull up bar, as shown and described.
FIG. 1 is a front-top-left perspective view of a pull up bar, showing my new design;
FIG. 2 is a back-top-left perspective view thereof;
FIG. 3 is a left elevational view thereof; the right elevational view being a mirror view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a front elevational view thereof, with the supports folded;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines showing of the mounting plates in FIGS. 1-8 are included for the purpose of showing environmental structure and form no part of the claimed design.