[object] Handle mallocation error in set_user_data

Should make bots happy.

src/hb-object.hh

diff --git a/src/hb-object.hh b/src/hb-object.hh
index fe44a0b..e2c2c33 100644
--- a/src/hb-object.hh
+++ b/src/hb-object.hh
@@ -69,7 +69,7 @@
       item = items.push (v);
       l.unlock ();
     }
-    return item;
+    return items.in_error () ? nullptr : item;
   }
 
   template <typename T>