[benchmark-subset] Free cached face upon exist

For better valgrind output.

perf/benchmark-subset.cc

@@ -116,6 +116,16 @@ static hb_face_t* preprocess_face(hb_face_t* face)
   return new_face;
 }
 
+static hb_face_t *cached_face;
+
+static void
+free_cached_face (void)
+{
+  hb_face_destroy (cached_face);
+  cached_face = nullptr;
+}
+
+
 /* benchmark for subsetting a font */
 static void BM_subset (benchmark::State &state,
                        operation_t operation,
@@ -126,7 +136,6 @@ static void BM_subset (benchmark::State &state,
 
   hb_face_t *face = nullptr;
 
-  static hb_face_t *cached_face;
   static const char *cached_font_path;
 
   if (!cached_font_path || strcmp (cached_font_path, test_input.font_path))
@@ -237,6 +246,10 @@ int main(int argc, char** argv)
 {
   benchmark::Initialize(&argc, argv);
 
+#ifndef HB_NO_ATEXIT
+  atexit (free_cached_face);
+#endif
+
   if (argc > 1)
   {
     num_tests = (argc - 1) / 2;