I claim the ornamental design for a safety cutter ring, as shown and described.
FIG. 1 is a perspective view showing my new design.
FIG. 2 is an opposite side perspective view showing my new design.
FIG. 3 is a front view showing my new design.
FIG. 4 is a rear view showing my new design.
FIG. 5 is a top view showing my new design.
FIG. 6 is a bottom view showing my new design; and,
FIG. 7 is a side view showing my new design.
The shading lines shown in the drawings represent the approximate three-dimensional contour of the design, and are not intended to indicate surface decoration.
The broken lines represent environment and form no part of the claim.