Fix invalid read when using LightOccluder2D

(cherry picked from commit 7870cf24f28ed93c9a105d31d454d02c1a79ad88)

servers/visual/visual_server_canvas.cpp

@@ -1141,7 +1141,7 @@ void VisualServerCanvas::canvas_light_occluder_set_polygon(RID p_occluder, RID p
 	ERR_FAIL_COND(!occluder);
 
 	if (occluder->polygon.is_valid()) {
-		LightOccluderPolygon *occluder_poly = canvas_light_occluder_polygon_owner.get(p_polygon);
+		LightOccluderPolygon *occluder_poly = canvas_light_occluder_polygon_owner.get(occluder->polygon);
 		if (occluder_poly) {
 			occluder_poly->owners.erase(occluder);
 		}