Fix one UBSan warning

Fixes https://github.com/harfbuzz/harfbuzz/issues/935

src/hb-font-private.hh

@@ -83,7 +83,11 @@ struct hb_font_funcs_t {
       HB_FONT_FUNCS_IMPLEMENT_CALLBACKS
 #undef HB_FONT_FUNC_IMPLEMENT
     } f;
-    void (*array[VAR]) (void);
+    void (*array[0
+#define HB_FONT_FUNC_IMPLEMENT(name) +1
+      HB_FONT_FUNCS_IMPLEMENT_CALLBACKS
+#undef HB_FONT_FUNC_IMPLEMENT
+		]) (void);
   } get;
 };