>1Byte UTF8 character issue if maxFieldSize is set

Detected at one of my clients.
They use polish special characters like ść
The length in TextRowProtocol is set to 2 for each of those characters so 4 in our example.
With this the current code

return new String(buf, pos, Math.min(maxFieldSize * 3, length), StandardCharsets.UTF_8) .substring(0, Math.min(maxFieldSize, length));

results into an ArrayOutOfBoundsError at the substring() call.

I added a test case and a quick fix for it.
See
https://github.com/mariadb-corporation/mariadb-connector-j/pull/156

Travis fails on environment issues. Nothing to do with my change. I can not rerun them....