BUG: Memory leak bug similar to a previous fixing commit a727596

https://github.com/shadowsocks/shadowsocks-libev/blob/e21f8248b64528b9a30a87331074a0eae6676225/src/jconf.c#L309

Similar to the memory leak that has been fixed with commit a727596faaa91a4459daad4cc5a6618c20ce6475, the argument returned by to_string will also get leaked after call to function parse_dscp.