The ornamental design of the medical injector, as shown and described.
FIG. 1 is a perspective view of the medical injector viewed from the front left showing my new design;
FIG. 2 is a perspective view thereof viewed from the rear left;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a front elevation view thereof; and,
FIG. 6 is a rear elevation view thereof, where the right side elevation view is a mirror image of the left side elevation view and the bottom plan view is a mirror image of the top plan view.
The broken lines show portions of the design that form no part of the claimed design.