Server crush

[Mathias]

Username can be max 16 characters long and only this are allowed

Code:

abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_

Any character can be used in a username on a Cuberite server, as long as the username is under 17 characters.

The character limit issue is reported here: https://github.com/cuberite/cuberite/issues/2545