fixing STL internal error checks firing (in MSVC2019 builds at least)

[GerHobbelt]

fixing STL internal error checks firing: "can't dereference value-initialized vector iterator", "cannot seek vector iterator after end".

Also fix: std::isspace() on Windows does not cope well with UTF8 or rather more precise: char values > 127 as those will become negative in that 8-bit char integer type, e.g. -89 and trigger the validation checks in the MSVC debug library, popping up yet another abort/retry/ignore debug report dialog box. For this the internal txt namespaced wrappers have been provided specifically...

(cherrypicked off https://github.com/GerHobbelt/clipp master branch; original commit tested with latest MSVC2019)