problem with wcstombs under windows

Hi,

the line mentioned gives problems under Windows because it always returns zero even for valid entries like "Root Entry". If the max length field (3rd parameterI is changed to len it works.

Regards
Otto

https://github.com/libxls/libxls/blob/3ea164845ff832e03117f1f2ba09304f88d03ec1/src/xlstool.c#L293