Prevent being able to set bone's parent as itself

(cherry picked from commit d1bacb53fd4a5c46b805af6eb7ccc93ff60bbafc)

scene/3d/skeleton.cpp

@@ -460,6 +460,7 @@ void Skeleton::set_bone_parent(int p_bone, int p_parent) {
 
 	ERR_FAIL_INDEX(p_bone, bones.size());
 	ERR_FAIL_COND(p_parent != -1 && (p_parent < 0));
+	ERR_FAIL_COND(p_bone == p_parent);
 
 	bones.write[p_bone].parent = p_parent;
 	process_order_dirty = true;