MySQL documents annouce the AuthSwitch packet is contains with two
component `auth_plugin_name` and `auth_data`, which `auth_data` is
a string[EOF] string. But in fact it will return a string[NUL] string
or can also say the string[EOF] is consist of a 20bytes string and a '\0'
byte. Now we just follow the document which use those 21bytes as salt
and that is not correct.
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