fix up Fills::MergeDownUni unit test #1615

src/tests/fills.cpp

@@ -434,7 +434,7 @@ TEST_CASE("Fills") {
     CHECK(0 == ncplane_cursor_move_yx(p3, 0, 0));
     // make sure glyphs replace glyps
     CHECK(0 < ncplane_putstr(p3, "🞵🞶🞷🞸🞹█▀▄▌▐"));
-    CHECK(0 == ncplane_mergedown_simple(p3, nullptr));
+    CHECK(0 == ncplane_mergedown_simple(p3, n_));
     nccell c3 = CELL_TRIVIAL_INITIALIZER;
     for(int i = 0 ; i < 10 ; ++i){
       CHECK(0 < ncplane_at_yx_cell(n_, 0, i, &cbase));