Make TSan happy with make_immutable()
diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index fca3c91..f095266 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -293,6 +293,8 @@
 {
   if (hb_object_is_inert (blob))
     return;
+  if (blob->immutable)
+    return;
 
   blob->immutable = true;
 }