The ornamental design for an autoinjector, as shown and described.
FIG. 1 is a front view of an auto injector in accordance with my new design;
FIG. 2 is another front view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom perspective view thereof; and,
FIG. 7 is a top perspective view thereof.
The dashed lines show the medication and mechanism for injection of the medication seen through the transparent or translucent main body and form no part of the claim.