Fix 64-bit compilation (at least on MSVC 2017, 15.3)

[janosvitok]

Otherwise warning is issued (assignment loses precision)

[OlafvdSpek]

Thanks! The warning is "warning C4267: 'initializing': conversion from 'size_t' to 'int', possible loss of data" though, precision is a floating point thingy. ;)

BTW, std::string::size_type would be even more correct but size_it is fine with me.