Fix adding bones with the same name after calling Skeleton3D.clear_bones()

(cherry picked from commit ea80213eee219e1553e8783f263915b3122b23f1)

scene/3d/skeleton_3d.cpp

@@ -543,6 +543,7 @@ bool Skeleton3D::is_show_rest_only() const {
 
 void Skeleton3D::clear_bones() {
 	bones.clear();
+	name_to_bone_index.clear();
 	process_order_dirty = true;
 	version++;
 	_make_dirty();