Prevent panic on malformed auth data

[timvaillancourt]

This PR resolves a panic in readAuthData when the function receives malformed data:

panic: runtime error: index out of range [129] with length 129

This occurs when the pos integer passed to readAuthData is greater than the length of data

This is causing a panic/crash of a MySQL proxy that uses github.com/siddontang/go-mysql/server whenever a vulnerability scanning tool ("Nessus") tries to test CVE-2006-1516 against the MySQL port

I don't have the full context on the CVE or what Nessus is testing, but this fix ensures a ER_HANDSHAKE_ERROR is returned instead of a panic if the pos is greater than the length of the data slice

[atercattus]

We got the same panic in our production :)

[timvaillancourt]

Thanks @atercattus!