The ornamental design for an injector, as shown and described.
FIG. 1 is a front, side perspective view of an injector showing my new design;
FIG. 2 is a rear, side perspective view thereof;
FIG. 3 is a right side perspective view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.