The ornamental design for a catheter, as shown and described.
FIG. 1 is a perspective view of a catheter showing our new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The parts shown with broken lines are not considered to be a part of the claimed design. The catheter is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.