advance refcnt only if it was used

src/raw_decoder.cpp

@@ -63,11 +63,12 @@ int zmq::raw_decoder_t::decode (const uint8_t *data_, size_t size_,
     int rc = in_progress.init ((unsigned char*)data_, size_,
                                shared_message_memory_allocator::call_dec_ref,
                                allocator.buffer(),
-                               allocator.create_refcnt() );
+                               allocator.provide_refcnt() );
 
     // if the buffer serves as memory for a zero-copy message, release it
     // and allocate a new buffer in get_buffer for the next decode
     if (in_progress.is_zcmsg()) {
+        allocator.advance_refcnt();
         allocator.release();
     }