Solution: explicitly set u.base.metadata to null

src/msg.cpp

@@ -239,6 +239,7 @@ int zmq::msg_t::close ()
         if (u.base.metadata->drop_ref ()) {
             LIBZMQ_DELETE(u.base.metadata);
         }
+        u.base.metadata = NULL;
     }
 
     //  Make the message invalid.
@@ -393,9 +394,11 @@ void zmq::msg_t::set_metadata (zmq::metadata_t *metadata_)
 void zmq::msg_t::reset_metadata ()
 {
     if (u.base.metadata) {
-        if (u.base.metadata->drop_ref ())
+        if (u.base.metadata->drop_ref ()) {
             LIBZMQ_DELETE(u.base.metadata);
         }
+        u.base.metadata = NULL;
+    }
 }
 
 bool zmq::msg_t::is_identity () const