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.
@@ -393,8 +394,10 @@ 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;
     }
 }
 
@@ -493,12 +496,12 @@ bool zmq::msg_t::rm_refs (int refs_)
     return true;
 }
 
-uint32_t zmq::msg_t::get_routing_id() 
+uint32_t zmq::msg_t::get_routing_id()
 {
     return u.base.routing_id;
 }
 
-int zmq::msg_t::set_routing_id(uint32_t routing_id_) 
+int zmq::msg_t::set_routing_id(uint32_t routing_id_)
 {
     u.base.routing_id = routing_id_;
     return 0;