Empty FormatArg() constructor doesn't initialize members

https://github.com/c42f/tinyformat/blob/46a6d778e7abafa057efc546197603d32719bfc5/tinyformat.h#L490

This confuses Coverity, and also looks unsafe:

![screenshot from 2017-10-02 09-25-09](https://user-images.githubusercontent.com/1741643/31097786-d5d7c5e0-a7c0-11e7-918b-00fdc8e51768.png)

Reference: https://github.com/RcppCore/Rcpp/issues/760