One more warning fix

Author: King_DuckZ

src/ChunkedReaderImpl.cpp

@@ -46,8 +46,15 @@ class ChunkedReaderImpl : public DataReader {
     }
 
     void Log(const boost::asio::const_buffers_1 buffers, const char *tag) {
+#if __cplusplus >= 201703L
+        [[maybe_unused]]
+#endif
         const auto buf_len = boost::asio::buffer_size(*buffers.begin());
 
+#if __cplusplus < 201703L
+        static_cast<void>(buf_len);
+#endif
+
         // At the time of the implementation, there are never multiple buffers.
         RESTC_CPP_LOG_TRACE_(tag << ' ' << "# " << buf_len
             << " bytes: "