JNLP3 challenge response generates invalid string encoding

Oh looky here

https://github.com/jenkinsci/remoting/blob/master/src/main/java/org/jenkinsci/remoting/engine/Jnlp3Util.java#L93

So the digest can generate any bytes you care... but not all byte sequences are valid UTF-8... thus you have a random chance of failure to validate... isn't that wonderful!