Minor

test/api/test-ot-math.c

diff --git a/test/api/test-ot-math.c b/test/api/test-ot-math.c
index e7a6fc8..b9fcb65 100644
--- a/test/api/test-ot-math.c
+++ b/test/api/test-ot-math.c
@@ -79,6 +79,9 @@
   hb_face_destroy (hb_face);
   hb_font_destroy (hb_font);
   FT_Done_Face (ft_face);
+  hb_face = NULL;
+  hb_font = NULL;
+  ft_face = NULL;
 }
 
 static void