The ornamental design for a safetybumper for a trailer tailgate, as shown and described.
FIG. 1 is an isometric bottom view of the safety bumper for a trailer tailgate in a state of use; the broken lines illustrating a trailer tailgate frame represent environment that forms no part of the claim;
FIG. 2 is an isometric top view of the safety bumper for a trailer tailgate of FIG. 1;
FIG. 3 is an isometric bottom view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a right side view thereof, the left side view being a mirror image thereof.