The ornamental design for a steellinestopper, as shown and described.
FIG. 1 is a perspective view of a steel line stopper;
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;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view of the steel line stopper in a state of use, wherein the broken line showing is a representation of automotive steel line.
The broken lines shown in the drawings illustrate environmental structure for the steel line stopper that forms no part of claimed design.