Prefix may not be properly filled

[kozlovic]

The RandomizePrefix function seems to be incorrect. The loop is using l as the condition to continue, however, there is no guarantee that l is going to be big enough that the loop will cover all 12 indexes of the prefix array:

https://github.com/nats-io/nuid/blob/master/nuid.go#L111

Actually, since maxPre (36^12) is used as the max for rand.Int(rand.Reader, big.NewInt(maxPre)), it is quite probable. A quick test showed that 2% of runs, i would end-up greater than 0 at the end of the loop. Should the condition had been i? or should maxPre be the minimum acceptable value for r, and therefore rand.Int() should be passed the max int64 value?