mysql_change_user() with long passwords throws an error

Followup from MDEV-38431

When COM_CHANGE_USER is sent with a password longer than 255 the client library returns an error. This is due to the fact that the code expects that the password length can always be encoded in one byte.
This is not the case for regular logins: there the code takes one byte for password lengths up to 251 and then reverts to normal variable length encoding for the length used by the protocol.