:hammer: fixed GCC warnings in #663
:hammer: fixed GCC warnings in #663

https://github.com/nlohmann/json/pull/663#issuecomment-318931884

nlohmann/json,kaidokert/json,kaidokert/json,kaidokert/json,theodelrieu/json,theodelrieu/json,nlohmann/json,nlohmann/json