Nobody ever checked the return value anyway, so just return the string
and use the stream state bits if necessary to find failures.

Doesn't need to be a member, make a standalone function

Rename it to read_zeroTerminated_uInt8s_AsO[U]String, stupid perhaps,
but *shrug*, unambiguous.

Drop misleading overloaded String variants use:
read_zeroTerminated_uInt8s_AsOString or
read_zeroTerminated_uInt8s_AsOUString

added a unit test, valgrinded it, found and fixed invalid read
in original implementation.