Fix use of gwbuf_length in mxs::Buffer

Backend should use empty() instead of length() to see if the buffer is
empty. The length of a buffer should always be valid to call, even on
empty buffers.

server/core/backend.cc

@@ -253,7 +253,7 @@ bool Backend::write_stored_command()
     mxb_assert(in_use());
     bool rval = false;
 
-    if (m_pending_cmd.length())
+    if (!m_pending_cmd.empty())
     {
         rval = write(m_pending_cmd.release());