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.

include/maxscale/buffer.hh

@@ -1099,7 +1099,7 @@ public:
      */
     size_t length() const
     {
-        return gwbuf_length(m_pBuffer);
+        return m_pBuffer ? gwbuf_length(m_pBuffer) : 0;
     }
 
     /**

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());