Fix nullptr de-reference failure if both blobs in a subset test are 0 in size

test/api/hb-subset-test.h

@@ -99,7 +99,10 @@ hb_subset_test_check (hb_face_t *expected,
   expected_blob = hb_face_reference_table (expected, table);
   actual_blob = hb_face_reference_table (actual, table);
   fprintf(stderr, "comparing %c%c%c%c, expected %d bytes, actual %d bytes\n", HB_UNTAG(table), hb_blob_get_length(expected_blob), hb_blob_get_length (actual_blob));
-  hb_test_assert_blobs_equal (expected_blob, actual_blob);
+
+  if (hb_blob_get_length (expected_blob) != 0 ||
+      hb_blob_get_length (actual_blob) != 0)
+    hb_test_assert_blobs_equal (expected_blob, actual_blob);
   hb_blob_destroy (expected_blob);
   hb_blob_destroy (actual_blob);
 }