advance refcnt only if it was used

src/v2_decoder.cpp

@@ -129,11 +129,12 @@ int zmq::v2_decoder_t::size_ready(uint64_t msg_size, unsigned char const* read_p
         // if the message will be a large message, pass a valid refcnt memory location as well
         rc = in_progress.init( (unsigned char*)read_pos, msg_size,
                                shared_message_memory_allocator::call_dec_ref, buffer(),
-                               create_refcnt() );
+                               provide_refcnt() );
 
         // For small messages, data has been copied and refcount does not have to be increased
         if (in_progress.is_zcmsg())
         {
+            advance_refcnt();
             inc_ref();
         }
     }