The ornamental design for a jar, as shown and described.
FIG. 1 is a perspective view of a jar showing my new design;
FIG. 2 is a front view thereof, a rear view being identical to the front view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a right side view thereof, a left side view being identical to the right side view; and,
FIG. 6 is a cross-sectional view thereof along the line 6-6 in FIG. 3.
The broken lines in the drawings showing the inside structure of the jar in FIG. 6 illustrate the portions of the jar that form no part of the claimed design.