The ornamental design for a container for a syringe, as shown and described.
FIG. 1 is a front perspective view of container for a syringe showing the claimed design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines illustrating a syringe depict environmental subject matter and form no part of the claimed design. The rest of the broken lines depict portions of the container for a syringe that form no part of the claimed design.