HPCC-12349 JSON outputUnicode outputs extra characters

JSON outputUnicode and outputUtf8 are handling byte count vs character count inconsistently when calling appendJSONStringValue.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Would a better fix be to pass in a maximum length to utf8CharLen(const byte *...), which defaulted to (unsigned)-1?