checkUTF8 check whether a given raw vector can be used as a
valid string encoded in UTF8.
checkUTF8(what, quiet = FALSE, charlen = FALSE, min.char = 1L)TRUE then the function will not fail
but report success/failure via its result, otherwise failures are
considered errors.TRUE then the function returns the
length of the longest byte sequence representing a character in the
file.charlen=FALSE: TRUE on success, FALSE if the
payload is invalid and quite=TRUE.If charlen=TRUE: positive integer corresponding to the
longest encoded sequence on success, negative integer on failure.