Make TSan happy with make_immutable()

src/hb-unicode.cc

diff --git a/src/hb-unicode.cc b/src/hb-unicode.cc
index bb1cd68..0e8b520 100644
--- a/src/hb-unicode.cc
+++ b/src/hb-unicode.cc
@@ -308,6 +308,8 @@
 {
   if (unlikely (hb_object_is_inert (ufuncs)))
     return;
+  if (ufuncs->immutable)
+    return;
 
   ufuncs->immutable = true;
 }