FIXING: sro() dies if we pass a code that is valid, but not existis.

If we pass a code that not existis, the response will not have a table, and the fist HTML::TreeBuilder::find will returns undef. The second find will die the script.

Ex: http://websro.correios.com.br/sro_bin/txect01$.Inexistente?P_LINGUA=001&P_TIPO=002&P_COD_LIS=RA189611162BR